Hertfordshire · Five districts · Studio in Hampstead, NW3
Renovating, extending and converting lofts and basements in five Hertfordshire districts, with design, engineering and building handled by the team at our Finchley Road studio.
This page gathers the checks that apply across all five districts: who decides an application, how far householder permitted development reaches, and the Green Belt, Chilterns, groundwater and flood questions to settle before design. The district pages then carry their own councils' plans and local policy in full.
Serving Hertfordshire from our Hampstead studio · NW3 6DN
020 7101 3168
Four districts
01 Radlett and Bushey
The Hertsmere guide works through the Core Strategy and the SADM Policies Plan, quotes Policy SADM 26 on Green Belt extensions, and covers the borough's basement guidance, heritage controls and brooks. Our Radlett and Bushey pages sit beneath it.
02 St Albans and Harpenden
The St Albans district guide reads from the council's newly adopted Local Plan: its Green Belt test and size guide, conservation areas, chalk groundwater and the shared building control service. It leads to our St Albans and Harpenden pages.
03 Rickmansworth and Chorleywood
The Three Rivers guide sets out the district's adopted plans, its Green Belt guidance for houses, rear-extension depths and five made neighbourhood plans. Rickmansworth and Chorleywood each have their own town page with services.
04 Cuffley and Brookmans Park
The Welwyn Hatfield guide covers the borough's adopted Local Plan, Policy SADM 34 for Green Belt extensions and the large excluded villages, with a section on Brookmans Park. Cuffley has its own town page.
First steps
Every Hertfordshire project is run from Finchley Road in Hampstead, NW3, where our studio is; we have no other office or branch. Our London team of in-house architects, designers and structural engineers, working with Hampstead Chartered Surveyors & Building Consultancy, takes on projects in Hertsmere, St Albans district, Dacorum, Three Rivers and Welwyn Hatfield, and comes to the property for the free feasibility visit.
Seven towns have their own pages: Radlett and Bushey in Hertsmere, St Albans and Harpenden in St Albans district, Rickmansworth and Chorleywood in Three Rivers, and Cuffley in Welwyn Hatfield. Each reads its district's policy against the town's own streets, designations and houses, and links the services we offer there.
The district council for the address decides the planning side: Hertsmere Borough Council, St Albans City and District Council, Dacorum Borough Council, Three Rivers District Council or Welwyn Hatfield Borough Council. For building regulations you can use the council's own provider, which is Hertfordshire Building Control in four of these districts and the St Albans and Watford shared building control service for St Albans district addresses, or appoint a private registered building control approver.
Ahead of the visit we run the address through a fixed set of lookups: the district and its adopted plan, statutory and local listing, conservation area and Article 4 coverage, the Green Belt and Chilterns boundaries, the flood map and the groundwater zones. The results tell us which consent route a design will need, and which of your ideas are worth drawing at all.
We plot the house against the district plan, heritage records, Green Belt, landscape, flood and groundwater layers, and note anything that narrows what can be built.
With you on site we look at structure, roof, drainage, levels and access, and say plainly which parts of the brief the constraints leave open.
A measured survey gives the existing drawings; our architects and designers build the scheme on them, and our structural engineers size beams, foundations and any retaining walls.
We prepare whatever the scheme needs, from a householder application or listed building consent to a lawful development certificate, then the building regulations package and any party wall notices.
Our site team protects the house and its neighbours, sequences the trades, calls building control out at each stage, and closes the job with the completion certificate and as-built drawings.
Sources: Hertfordshire Building Control: FAQs; Hertfordshire Building Control: Regulatory information; St Albans City and District Council: Building Control - Shared Service; GOV.UK: Building regulations approval - when you need approval
Who decides
Two councils can be involved in one Hertfordshire renovation. The planning application goes to the district or borough council: Hertfordshire Building Control's own guidance tells anyone with a planning enquiry to contact their local council and its planning department, and national guidance says the local planning authority decides an application on its development plan.
Decided by Hertsmere Borough Council against the Hertsmere Core Strategy (adopted January 2013) and Elstree Way Corridor Area Action Plan (adopted July 2015) and Site Allocations and Development Management Policies Plan (adopted 23 November 2016). Our Hertsmere page sets the plan out.
Decided by St Albans City and District Council against the St Albans Local Plan 2026 (Parts A and B) (adopted 9 September 2026), which wholly replaces the 1994 plan. Our St Albans district page explains it.
Decided by Dacorum Borough Council against the Dacorum Core Strategy (adopted September 2013) and Dacorum Site Allocations DPD (adopted July 2017) and saved policies of the Dacorum Borough Local Plan 1991-2011 (adopted April 2004).
Decided by Three Rivers District Council against the Three Rivers Core Strategy (adopted 17 October 2011) and Development Management Policies Local Development Document (adopted July 2013) and Site Allocations Local Development Document (adopted 2014). Our Three Rivers page sets them out.
Decided by Welwyn Hatfield Borough Council against the Welwyn Hatfield Local Plan 2016-2036 (adopted 12 October 2023), which replaced the saved 2005 District Plan policies. Our Welwyn Hatfield page explains it.
Hertfordshire County Council handles the highway. Its residential dropped kerbs policy says you are not allowed, under the Highways Act 1980, to drive over a pavement or verge unless a dropped kerb has been approved and installed by the county council, and Hertsmere's own guidance confirms that the county's Highways department carries out the work.
Planning permission is normally needed for a dropped kerb off a classified road (an A, B or C road), you may need it for a new or extended driveway too, and the county will not accept a kerb application until any permission is in place. The policy asks for a driveway at least 5 metres deep and 2.7 metres wide, treats highway safety as the main consideration, and warns that a neighbour's existing dropped kerb does not mean yours will be approved.
We settle the parking layout, the paving and the crossover with the planning drawings, so the application, the kerb request and the driveway all describe the same arrangement.
Sources: Hertfordshire Building Control: FAQs; GOV.UK: Planning permission (printable guide); Hertsmere Borough Council: Local Plan 2012-2027 Core Strategy; Hertsmere Borough Council: Local Plan 2012-2027; Hertsmere Borough Council: Site Allocations and Development Management Policies Plan; St Albans City and District Council: Current Local Plan and SPDs (web page); Dacorum Borough Council: CD09.1 Dacorum Local Development Scheme (Oct 2024); Dacorum Borough Council (Let's Talk Dacorum): Dacorum Local Plan to 2041 News and Updates, page 4 (older items); Three Rivers District Council: Development Management Policies LDD (adopted July 2013); Three Rivers District Council: Development Plan (Current Local Plan); Welwyn Hatfield Borough Council: Local Plan (Adopted Local Plan - October 2023); Hertfordshire County Council: Residential Dropped Kerbs Policy, version 7 (8 May 2026); Hertsmere Borough Council: Dropped Kerbs and Crossovers
National rules
Some householder work needs no application, because national law, in the shape of the General Permitted Development Order 2015, already allows it. In Schedule 2, Part 1, Class A lets a house be enlarged, improved or otherwise altered and Class B lets its roof be added to or altered, each within the limits and conditions the Order writes out.
Those rights are narrower on article 2(3) land, a term the Order defines through Schedule 1, Part 1. That part lists any area of outstanding natural beauty and conservation areas (designated under the 1990 Listed Buildings Act, section 69). In law the Chilterns National Landscape is an Area of Outstanding Natural Beauty, so the article 2(3) limits reach a house inside it just as conservation area status would.
Paragraph A.2 removes three things: an extension beyond a side wall of the original house; an extension of more than one storey beyond its rear wall; and cladding of the exterior in stone, artificial stone, pebble dash, render, timber, plastic or tiles.
Paragraph A.1(g)'s longer single-storey rear extensions are only for houses off article 2(3) land and outside any site of special scientific interest.
Paragraph B.1(f) withdraws Class B altogether for a house on article 2(3) land. A rear dormer on a conservation area or Chilterns house therefore goes through a planning application rather than permitted development.
Class E gives no right, on article 2(3) land, to put a garden building, enclosure, pool or container in the strip beside the house, between its side wall and the plot boundary.
Anywhere, a new or replaced hard surface in front of the house, between its front wall and the highway, covering more than 5 square metres must be porous or drain to a permeable area within the plot.
Local controls can remove rights too. National guidance says a local planning authority can withdraw them by attaching a condition to an earlier permission or by making an article 4 direction, so the planning history of the house counts as much as the map. Each council's directions are covered on our district pages.
Where it is not clear whether a project is permitted development, a lawful development certificate gives a legally binding decision from the local planning authority. When the position depends on a measurement, an old consent or the line of the original house, we apply for one before building rather than rely on a reading of the Order.
Sources: legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 2 Part 1, development within the curtilage of a dwellinghouse (latest available revised); legislation.gov.uk: Town and Country Planning (General Permitted Development) (England) Order 2015 (SI 2015/596), article 2 (latest available revised); legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 1 Part 1, Article 2(3) land (latest available revised); Chilterns National Landscape (Chilterns Conservation Board): Chilterns National Landscape Boundary Review (page last updated 25 June 2025); GOV.UK (Planning Practice Guidance): When is permission required?
Green Belt
Every Green Belt decision starts from national policy. The National Planning Policy Framework published on 17 August 2026 says development there is inappropriate unless it falls within one of the categories in policy GB7, and one category is the extension or alteration of an existing building that will not result in a disproportionate increase in size compared to the original building.
Footnote 40 sets the yardstick: for this purpose the original building means the building standing on 1 July 1948, and for anything constructed later, the building as originally built. How large an addition can be before it becomes disproportionate is then a matter for each district's adopted policy, and the councils word theirs differently.
An extension or alteration “must not individually or cumulatively be disproportionate to the original building”; the additions must also be subordinate to it. The Hertsmere page sets out the full test.
Extensions “should not create a building that is disproportionately larger or of significantly different character than the original building”, and Table 3.3 gives a guide to the size normally permitted. The district page quotes the table.
“Extensions to buildings in the Green Belt that are disproportionate in size (individually or cumulatively) to the original building will not be permitted.” Guidance of August 2003 on extensions to dwellings in the Green Belt adds figures. The Three Rivers page quotes them.
Applicants must show extensions would not result, individually or cumulatively, in disproportionate additions over the original building “in terms of bulk, scale, height or massing”. Brookmans Park and Cuffley are large excluded villages, not in the Green Belt. The Welwyn Hatfield page explains both.
Because the five tests are worded differently, we never carry a figure or a rule of thumb from one district into another. For a Green Belt house we rebuild the planning history first: the original 1948 building or, if later, the house as first built, then every addition and outbuilding since, measured and drawn, so the scheme is tested against the district's own words before a design takes hold.
Sources: MHCLG (GOV.UK): National Planning Policy Framework (published 17 August 2026, last updated 29 September 2026); MHCLG (GOV.UK): National Planning Policy Framework, Green Belt chapter, policies GB1 to GB8 (August 2026); Hertsmere Borough Council: Site Allocations and Development Management Policies Plan (adopted November 2016); St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); Three Rivers District Council: Development Management Policies LDD (adopted July 2013); Welwyn Hatfield Borough Council: Welwyn Hatfield Local Plan 2016-2036 (adopted 12 October 2023); Hertsmere Borough Council: Site Allocations and Development Management Policies Plan; St Albans City and District Council: Current Local Plan and SPDs (web page); Three Rivers District Council: SPG No.3 Extensions to Dwellings in the Green Belt (August 2003); Welwyn Hatfield Borough Council: Local Plan (Adopted Local Plan - October 2023)
Protected landscape
The Chilterns National Landscape is legally an Area of Outstanding Natural Beauty: land protected by the Countryside and Rights of Way Act 2000. Natural England stopped its review of the Chilterns boundary for lack of funding, and says the existing boundary is unaffected and remains in place.
The district's Regulation 19 draft plan says approximately 546ha of the National Landscape lies within Three Rivers. The Chorleywood Neighbourhood Plan places the parish on the boundary of the Chilterns AONB, with Chorleywood House Estate inside it. Our Chorleywood page looks at what that means for a house.
Dacorum's pre-submission plan describes Berkhamsted as located in the setting of the Chilterns National Landscape, which lies to the north.
The 2026 Local Plan says the National Landscape lies to the north of St Albans District. Our St Albans and Harpenden pages cover the two towns.
The setting matters as well as the line. National policy asks for development within the setting of a Protected Landscape to be sensitively located and designed to avoid or minimise adverse impacts on it, and for development inside one to be limited in scale and extent.
For any address near the Chilterns we check the boundary on Natural England's mapping before the first visit. Inside it we design to the narrower permitted development rights from the start; just outside it, we treat the view back from the National Landscape as part of the brief, choosing roof forms, materials and glazing with that view in mind.
Sources: Chilterns National Landscape (Chilterns Conservation Board): Chilterns National Landscape Boundary Review (page last updated 25 June 2025); Natural England (GOV.UK): Areas of outstanding natural beauty (AONBs): designation and management (last updated 19 November 2024); Natural England: Chilterns AONB Boundary Review - Natural Beauty Assessment (NECR691), publication record (uploaded 2026/06/26); Three Rivers District Council: Three Rivers Local Plan 2026-2042 Part I (Regulation 19, July 2026); Chorleywood Parish Council / Three Rivers DC: Chorleywood Neighbourhood Plan (linked as Updated July 2026); Dacorum Borough Council: CD01.1 Dacorum Local Plan to 2041 Pre-Submission Version (Nov 2024); St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); MHCLG (GOV.UK): NPPF Chapter 19, Conserving and enhancing the natural environment (N1-6) (August 2026); MHCLG (GOV.UK): NPPF Annex B, Glossary (August 2026)
Chalk and groundwater
Groundwater shapes what can happen below ground in much of our area. Three Rivers' flood assessment says the entire district is covered by groundwater source protection zones, reflecting an underlying chalk aquifer; St Albans' Local Plan says the whole district is covered by Source Protection Zones; and Dacorum's assessment says large areas of the borough lie within them.
Further east and south the picture changes. Hertsmere's 2023 assessment describes the borough as lying on the boundary between the chalk of Hertfordshire to the north and the London Clay and Reading Beds to the south, and warns developers of several source protection zones; Welwyn Hatfield's plan says the southern half of its borough is underlain by various minor aquifers.
A groundwater SPZ1 is the area immediately around an abstraction point for domestic supply or food production. Within it, the Environment Agency will normally object in principle to any planning application for development that may physically disturb an aquifer, and its own examples of disturbance include foundation development, developments that require piling and basement excavations. The zones can be viewed on the Magic map.
Chalk also brings flooding from below: the Agency notes that groundwater flooding is most common where the underlying bedrock is chalk. The district assessments respond in their basement advice. St Albans, Dacorum and Three Rivers advise that habitable basements in Flood Zone 3 should not be permitted, Dacorum and Three Rivers extend that to land at high risk of surface water flooding, St Albans says basements should be avoided in high groundwater zones, and Hertsmere's design guide finds residential basements not appropriate in Flood Zone 3.
That is why every basement page we publish for Hertfordshire is marked provisional. Before any basement or deep foundation design we commission a ground investigation with groundwater readings and check the source protection zones; those results decide whether excavation is sensible, how the ground would be retained and whether piling is acceptable at all. We never promise a basement before they are in.
Sources: Three Rivers District Council (JBA): Level 1 Strategic Flood Risk Assessment 2026; St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); Dacorum Borough Council: CSN03.1 Dacorum Level 1 Strategic Flood Risk Assessment Final Report (Sep 2024); Hertsmere Borough Council / AECOM: Level 1 Strategic Flood Risk Assessment (2023); Welwyn Hatfield Borough Council: Welwyn Hatfield Local Plan 2016-2036 (adopted 12 October 2023); Environment Agency (GOV.UK): Groundwater source protection zones (SPZs) (last updated 1 August 2024); Environment Agency: The Environment Agency's approach to groundwater protection (LIT 7660; under review); St Albans City and District Council (examination library ENV 001): South West Hertfordshire Strategic Flood Risk Assessment - Report - Final Draft (2018); Hertsmere Borough Council: Draft Planning and Design Guide SPD Part D, Guidelines for High Quality Sustainable Development (October 2016 draft, used for decisions since September 2016)
Flood risk
Flood risk information for a planning application comes from the Environment Agency's Flood Map for Planning. The Agency also says the owner of a straightforward, low-risk scheme such as a house extension may be able to write the flood risk assessment themselves.
The rivers Lea, Colne and Ver flow through St Albans district, which counts them among only 210 chalk streams in the world, with the Ver running through St Albans City. Three Rivers takes its name from the Colne, the Gade and the Chess.
Hertsmere's 2023 assessment lists four Environment Agency flood warning areas in the borough, among them the Radlett Brook at Radlett and the River Colne near Bushey Hill Farm. Welwyn Hatfield's plan finds its built-up areas at relatively low risk from rivers, with surface water and groundwater still risks.
Our first job on any address is to print its flood map; the district's own Strategic Flood Risk Assessment is then read for that street before floor levels, thresholds or drainage are fixed. Where surface water rather than a river is the risk, we design the falls, the soakaway and the threshold of the extension together, and the district pages say what each assessment asks for.
Sources: Environment Agency (GOV.UK service): Get flood risk information for planning in England (Flood map for planning); St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); Three Rivers District Council: Three Rivers Local Plan 2026-2042 Part I (Regulation 19, July 2026); Hertsmere Borough Council: New Local Plan supporting studies; Hertsmere Borough Council / AECOM: Level 1 Strategic Flood Risk Assessment (2023); Welwyn Hatfield Borough Council: Welwyn Hatfield Local Plan 2016-2036 (adopted 12 October 2023)
Sign-off
Planning permission is one thing and building regulations approval another, and one project can need both. In four of our five districts the council's service is Hertfordshire Building Control, which names itself the Building Control Authority for Welwyn Hatfield, Hertsmere, Broxbourne, Stevenage, East Herts, North Herts, Three Rivers and Dacorum.
St Albans district is the exception. St Albans City and District Council and Watford Borough Council work together on building control, so regulations for a house in St Albans or Harpenden go to the St Albans and Watford shared building control service, not to the provider the other four districts use.
The council's service is not compulsory. In any district a private registered building control approver may take the work on instead; once appointed, the approver tells the local authority about the work, which is called giving an initial notice.
Building control's role has narrowed. Hertfordshire Building Control explains that it is no longer considered part of the design team, that its role is centred on inspection and enforcement, and that building control bodies can no longer provide design advice; designers and contractors must make sure the work complies.
That is the gap our in-house team fills: the compliant drawings and calculations are produced in-house by our architects and structural engineers, our site managers arrange every inspection the surveyor wants, and the handover file includes the completion certificate whichever building control body you pick.
Sources: GOV.UK: Building regulations approval - when you need approval; Hertfordshire Building Control: Regulatory information; Hertfordshire Building Control: FAQs; St Albans City and District Council: Building Control - Shared Service; St Albans City and District Council: Building control (web page); GOV.UK: Building regulations approval (printable guide)
Across the boundary
Postcodes do not follow the county line. Postcode records show the WD23 district taking in addresses in Harrow, Hertsmere and Watford, WD6 in Barnet and Hertsmere, HA6 in Hillingdon and Three Rivers, and EN6 in Broxbourne, Enfield, Hertsmere and Welwyn Hatfield. On Harrow's own site, its conservation areas are grouped under headings that include Stanmore and Edgware.
Houses on the London side belong to our London pages rather than these county ones. Stanmore, Edgware, Pinner, Northwood, Harrow on the Hill, High Barnet, Totteridge, Mill Hill, Whetstone and Cockfosters are all London areas. If a house sits close to the line, send us the address and we will check which council covers it first.
Sources: postcodes.io (ONSPD-derived): outcode WD23; postcodes.io (ONSPD-derived): outcode WD6; postcodes.io (ONSPD-derived): outcode HA6; postcodes.io (ONSPD-derived): outcode EN6; London Borough of Harrow: Conservation areas and Article 4
Our Hampstead base, serving Hertfordshire
Our studio is in Hampstead on Finchley Road, at NW3 6DN. We serve Hertfordshire from this office. Bring us the practical questions about your house, flat or bathroom and the result you want to achieve.
Contact the studio to discuss your brief and arrange the next step. If you already have drawings, a lease or planning correspondence, tell us what is available.
Book a free feasibility visitOur work
The example we point to is in London. It is worth reading for the way one brief brought several of the services on these pages, from the basement up to the loft, into a single programme of work under one team.
Our renovation guides take each stage further, from the first survey to handover, and our London pages on basement conversions, loft conversions, house extensions, house refurbishment, kitchen renovation and bathroom renovation set out the way each type of job is run. How we work introduces the team, and all areas places Hertfordshire on the same map as our London areas.
Outside Hertfordshire, and in London rather than Hertfordshire: Project example: Belsize Park, NW3. That project of ours is a Victorian townhouse of five storeys; its case study records a basement conversion, a rear extension and loft work, carried out together with a full refurbishment of the interior.
Sources: Hampstead Renovations: Belsize Park Gardens, NW3 case study
Hertfordshire questions
Yes. Our London team of in-house architects, designers and structural engineers, working with Hampstead Chartered Surveyors & Building Consultancy, takes on projects in Hertsmere, St Albans district, Dacorum, Three Rivers and Welwyn Hatfield from the studio on Finchley Road in Hampstead, NW3; there is no office in the county. The district council decides planning, and building regulations can be handled by the council's building control provider or by a private registered building control approver of your choosing.
The district or borough council for the address: in the five districts these pages cover, that is Hertsmere Borough Council, St Albans City and District Council, Dacorum Borough Council, Three Rivers District Council or Welwyn Hatfield Borough Council. Each decides applications against its own adopted plan. Hertfordshire County Council looks after the highway, including dropped kerbs.
Hertfordshire County Council. Its policy says you may not drive over a pavement or verge unless a dropped kerb has been approved and installed by the county council, and Hertsmere's guidance confirms the county's Highways department does the work. Planning permission is normally needed first if the road is classified (A, B or C), and the county will not accept the kerb application until it is granted. The driveway must be at least 5 metres deep and 2.7 metres wide.
Only some addresses are. Three Rivers' draft plan says approximately 546ha of the National Landscape lies in the district and the Chorleywood Neighbourhood Plan puts the parish on its boundary; Dacorum describes Berkhamsted as in its setting; and St Albans' plan says it lies to the north of the district. Inside the boundary the house counts as article 2(3) land: Class B roof additions, side extensions, two-storey rear extensions and render, timber or tile cladding are then all outside permitted development. In the setting, national policy asks for sensitive siting and design. We check the boundary for every address.
Each district applies its own adopted policy: SADM 26 in Hertsmere, LG9 with its size table in St Albans district, DM2 with the SPG3 guidance in Three Rivers, and SADM 34 in Welwyn Hatfield. National policy supplies the common starting point, that an extension must not be disproportionate to the original building as it stood on 1 July 1948 or as first built, but the districts word their tests differently and some add their own size guidance. Each district page quotes its policy.
In Hertsmere, Dacorum, Three Rivers and Welwyn Hatfield the council's service is Hertfordshire Building Control. In St Albans district it is the St Albans and Watford shared building control service, which the council runs with Watford. Any district also lets you bring in a private registered building control approver instead, and the approver notifies the local authority of the work with an initial notice.
Print the address's Flood Map for Planning from the Environment Agency first. Next comes the district's own Strategic Flood Risk Assessment, and the groundwater source protection zones on the Magic map. Three Rivers says its whole district is covered by these zones, and St Albans says the same of its district. Within an SPZ1 the Environment Agency will normally object in principle to development that may physically disturb an aquifer, and the district assessments advise against habitable basements in Flood Zone 3. We commission a ground investigation before any basement design.
See how Hertfordshire sits alongside the London areas on our areas page.
Every district page we publish for Hertfordshire.