St Albans district · Hertfordshire
House extensions, lofts, basements and full refurbishment across St Albans district, with the design, engineering and building handled by a single Finchley Road team.
This is the district reference that our St Albans and Harpenden pages point back to: the Local Plan adopted in September 2026, its Green Belt size guide, conservation areas and Article 4 streets, the chalk groundwater under every address, the Ashridge mitigation strategy and who signs off the building work. Each section lists the council or national document it relies on.
Serving St Albans District from our Hampstead studio · NW3 6DN
020 7101 3168
St Albans district towns
01 Seven services
The St Albans page takes the district rules below down to the city's own conservation areas, Article 4 streets and groundwater record. It links our St Albans pages on kitchens and bathrooms, extensions, lofts, basements, and the refurbishment of houses and flats.
02 Six services
Harpenden's hub reads the same Local Plan alongside the town's neighbourhood plan and the flood picture along the Lee at Batford. From there you reach six Harpenden service pages, each written for houses in the town.
The plan in force
Applications across the district are now decided under the St Albans Local Plan 2026 (Parts A and B) (adopted 9 September 2026). It was adopted at an Extraordinary Council Meeting, it covers the period 2024 to 2041, and it wholly replaces the District Local Plan Review 1994.
The examination formally closed when the Inspectors' Report reached the council on Friday, 21 August 2026. The Inspectors concluded that, with their recommended Main Modifications, the plan is sound, legally compliant and capable of adoption, and the council notes only small changes from the modifications it consulted on between 13 April and 26 May 2026.
The council's own page puts the rule simply: planning applications have to be decided in line with the Development Plan unless there is a very good reason not to. Here it includes Parts A and B of the Local Plan and the made neighbourhood plans listed further down this page, which join the Development Plan once made.
Older guidance went with the old plan. The 1994 plan and its supplementary documents are no longer part of the Development Plan, and the guidance that accompanied it no longer applies, including the May 2004 supplementary planning guidance on residential extensions and replacement dwellings in the Green Belt. The council keeps those documents online for reference only.
Work on the next plan has begun. The planning policy page now refers to the next Local Plan (2029) and lists a scoping consultation for it alongside a 2026 call for sites. Nothing in that work is policy yet, so it plays no part in the advice on this page.
For each address we begin with the Policies Map and the council's constraints layers rather than with any memory of the 1994 policies. Green Belt, conservation area, Article 4, listing and flood zone are confirmed before the first sketch, and the design statement quotes the 2026 policy references that apply.
Sources: St Albans City and District Council: Current Local Plan and SPDs (web page); St Albans City and District Council: Adoption Statement for the St Albans City & District Local Plan 2026 (notice dated 10 September 2026); St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); St Albans City and District Council: Neighbourhood Planning (web page); St Albans City and District Council: Planning Policy (web page)
Green Belt
The majority of the district is designated Green Belt, and an extension or a replacement house there is tested against Policy LG9 (Extension or Replacement of Buildings in the Green Belt), which opens by saying that extensions to existing buildings will be assessed in accordance with national policy, then adds its own tests.
“Extensions should not create a building that is disproportionately larger or of significantly different character than the original building in terms of scale or visual impact.”
The original building is the building as it existed at 1st July 1948 or, if built after that date, as originally built. Previous and proposed extensions are assessed together, and “the increase in both volume and floor space over the original building will be considered.”
“Replacement buildings should be in the same use and not materially larger than the buildings they replace.” They should occupy the site of the existing building or stand as close to it as possible.
Where a replacement building exceeds the size of the existing one, permitted development rights may be removed by condition if needed to prevent further harm to Green Belt openness.
The supporting text sets out the criteria an extension is read against. Single storey side extensions should not close valuable gaps between buildings, and two storey side extensions may be resisted. Porches are generally acceptable. Rear extensions are “generally viewed more favourably than side / front extensions”. Significantly raising the eaves, the ridge or the bulk of a roof will normally be taken to create a building of different character, and the visibility of a proposal from public viewpoints is a material consideration.
Table 3.3 then gives “a guide to the size of extensions that will normally be permitted under Policy LG9”, graded by how well a scheme performs against those criteria. For an extension, the additional size is measured against the original building; for a replacement building, against the existing one.
“Less than 90m³ increase in cubic content or Less than 20% increase in floorspace (gross) subject to a maximum increase in cubic content of 300m³ (whichever is greater)”
“No more than 135m³ increase in cubic content or No more than 30% increase in floorspace (gross) subject to a maximum increase in cubic content of 300m³ (whichever is greater)”
“No more than 180m³ increase in cubic content or No more than 40% increase in floorspace (gross) subject to a maximum increase in cubic content of 300m³ (whichever is greater)”
“Above the top of the ranges, or maximum, applicable to Well performing proposals; assessed on a case-by-case basis”
Two cautions sit beneath the table. A judgment is made case by case on how the type of extension and its visibility from public viewpoints affect the openness and character of the Green Belt, and some roof or porch extensions can fall within the size limits yet still be unacceptable under the design policies in Chapter 12.
Our first job for a house in the Green Belt is the 1948 baseline. The planning history, old drawings and the fabric itself are used to separate the original building from every later addition, and both volume and gross floorspace are measured. The scheme is then scored against each criterion and each row of the table before it is shown to anyone, so the planning statement can set the arithmetic out line by line.
Sources: St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026)
Heritage
There are 19 conservation areas in the district. Each has a character statement describing its significance, character and appearance, and the council uses those statements when it assesses planning applications.
The St Albans Conservation Area Character Statement is large enough to be divided into a series of character areas, each with its own map, among them Verulamium, the Abbey and its precincts, Fishpool Street and St Michael's Village, Holywell Hill and Sopwell Lane, and Clarence Park. The planning constraints layer of the council's District Mapping Service, or a My St Albans account, shows whether an address is in a conservation area.
Policy HE1 asks for a heritage statement, proportionate to the scale of the proposal, with any scheme affecting a designated heritage asset or its setting. On roofs it is specific: dormers may be supported where they are appropriately sized, avoid terracing effects and suit the local roofscape, while flat roof dormers and crown roofs “will only be supported where they are characteristic of the local area”. Materials should be good quality and natural, with good long-term weathering characteristics.
Conservation area status also restricts some work that would otherwise need no permission, and planning permission is required for relevant demolition. Where ordinary permitted changes threaten a place, the council has used Article 4 directions, explaining the risk as “the gradual erosion of character through unsympathetic external alterations to buildings and the streetscene”. Its heritage page names four main areas: Verulam and Fishpool Street, Sopwell Lane and Albert Street, Cunningham Avenue, and Childwickbury. In them, planning permission must be sought for some works which would normally count as permitted development, and the Article 4 layer of the District Mapping Service shows the boundaries. Other directions are listed separately, including one for The Hawthornes, Lower Luton Road, in Harpenden.
Every locally listed building in the district stands within a conservation area. Local listing leaves the need for planning permission as it was, but the status is weighed whenever an application is determined, and where such a building has been extended, subdivided or is a block of flats the listing covers the whole building. Policy HE2 asks that special features contributing to a non-designated asset's significance are retained.
Two more policies bear on older houses. HE8 supports work to improve the environmental performance of heritage assets and buildings in conservation areas where it is designed to preserve or enhance their significance and does not harm them through inappropriate interventions, because, in the plan's words, traditionally constructed buildings have different technical requirements and inappropriate retrofitting can cause long-term problems. The district also sits on deep history: the Roman city of Verulamium was the third largest town in Roman Britain, and Policy HE3 requires, where appropriate, a desk-based assessment or field evaluation where proposals are on or next to sites of known archaeological interest.
In a conservation area we draw from a measured survey of the elevations and roof, read the character statement for the street before choosing a dormer or a facing material, and write the heritage statement from those findings. Where insulation, secondary glazing or new services are planned for an older house, the existing walls, floors and roof are understood first and the specification follows from them.
Sources: St Albans City and District Council: Conservation areas (web page); St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); St Albans City and District Council: Article 4 Direction areas (web page); St Albans City and District Council: Locally listed buildings (web page)
Groundwater
The whole district is covered by Source Protection Zones: Inner Zone 1, Outer Zone 2 or Total Catchment Zone 3. Policy HW4 requires development to take account of the advice for the zone it lies within, and the plan notes that certain activities and types of development are not appropriate within the zones, especially Inner Zone 1.
Chalk shapes the district's water. The Lea, the Colne and the Ver are three of only 210 chalk streams in the world, and the Ver flows through the city. The 2018 flood assessment, prepared jointly with neighbouring councils including Three Rivers and Dacorum, finds groundwater flood risk concentrated in the floodplains of the Lee, Ver and Colne and of the Butterwick and Ellen Brooks, where the chalk and gravel can hold groundwater at or just below the surface.
The assessment names southern St Albans, Marshalwick, Redbourn, Batford and Wheathampstead as the settlements at highest risk of groundwater flooding. Its record of past events includes groundwater flooding in Fishpool Street in December 2000, in Beverly Gardens in April 2001 and in Harper Lane in February 2006.
“Habitable uses of basements within Flood Zone 3 should not be permitted, whilst basement dwellings in Flood Zone 2 will be required to pass the Exception Test.”
Pumps in basements may be acceptable as a resilience measure when existing buildings are redeveloped, but for new development that is not an acceptable solution “and basements should be avoided in high groundwater zones.”
“Obstruction of sub-surface flows by buried services and basements should be avoided.” For groundwater risk, one option it names is building design with finished floor levels raised 300mm above the 1 in 100-year plus climate change water level.
In the Green Belt a basement is a planning question too. LG9 treats wholly subterranean basements with no external impact as generally acceptable, but any part that protrudes above ground where levels change, and features such as lightwells, sunken terraces and railings, are assessed in the same way as front and rear extensions, and the intensification of the use of the site is considered as well.
We never promise a basement here before the ground has been investigated. What decides one is the flood zone of the address, the groundwater level found by boreholes and monitoring, the source protection zone and, on a Green Belt plot, how much of the work shows above ground. Our structural engineers design retaining walls and waterproofing only once those answers are known, and if they point against a basement we say so. The St Albans and Harpenden basement pages work through those checks town by town.
Sources: St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); St Albans City and District Council (examination library ENV 004): South West Hertfordshire SFRA Appendix C - Understanding Flood Risk in St. Albans - Final Draft (2018); St Albans City and District Council (examination library ENV 001): South West Hertfordshire Strategic Flood Risk Assessment - Report - Final Draft (2018)
Flood risk
Three principal rivers cross the district. The Lee flows through Batford, Harpenden, in the north; the Ver runs through the central band from north west to south south-east; and the Colne follows the south-easterly boundary, where the Ver meets it at Bricket Wood.
The steep Chiltern dip slope keeps the floodplains of the Lee and Ver narrow, so Flood Zones 2 and 3 stay close to the channels. Along the Ver, East Redbourn and parts of south-west St Albans lie in the flood zones, which grow as the river passes through the city; on the Lee, small sections of north Batford and north-east Wheathampstead are in Flood Zones 2 and 3. The Colne, passing through Colney Heath and London Colney, forms the most significant floodplain in the district.
Surface water is a separate risk from the rivers: the assessment predicts surface water flooding in Harpenden in a 1 in 30-year rainfall event. Policy NEB8 asks minor and householder development to incorporate sustainable drainage where applicable, and the flood assessment warns that infiltration drainage can raise groundwater levels locally and may increase flood risk on or off the site.
Before floor levels are fixed we print the Environment Agency flood map and the surface water layers for the address. The drainage design is then tested against the soil and groundwater actually found in the trial pits, rather than assuming a soakaway will work.
Sources: St Albans City and District Council (examination library ENV 004): South West Hertfordshire SFRA Appendix C - Understanding Flood Risk in St. Albans - Final Draft (2018); St Albans City and District Council: St Albans Local Plan Part A (adopted 9 September 2026); St Albans City and District Council (examination library ENV 001): South West Hertfordshire Strategic Flood Risk Assessment - Report - Final Draft (2018)
Protected woodland
Ashridge Commons and Woods, a part of the Chilterns Beechwoods Special Area of Conservation, is under increasing visitor pressure from the surrounding areas. A 12.6km Zone of Influence drawn around it covers part of St Albans district, and within that buffer the plan says the council is legally required not to issue decisions until appropriate mitigation is secured through a Mitigation Strategy.
The joint mitigation strategy decides which applications that touches. Householder applications are screened out, as having no bearing on recreational pressure at Ashridge, and replacement dwellings are screened out too. Any proposal that delivers a net increase in residential dwellings within the zone is screened in. An extension or residential ‘granny’ annexe sits between the two: it is decided case by case, and the strategy says the answer “depends if it functions as a separate unit to the main dwelling.”
Where a scheme does add a home inside the zone, Strategic Policy SP10 expects a contribution to the Strategic Access Management and Monitoring Strategy and either a new Suitable Alternative Natural Greenspace or a contribution to an existing one. The strategy expects screened-out work that still adds visitor pressure to be rare, and asks applicants who are unsure to engage early with the council or with Natural England.
The Chilterns National Landscape lies to the north of the district rather than within it. Policy NEB11 still applies where relevant, and asks that development in the setting of a nationally designated landscape is sensitively located and designed to avoid or minimise adverse impacts on it.
When a brief includes an annexe, we settle at the outset whether it will work as part of the house or as a separate unit, because that can change how the application is screened, and the intended use is recorded on the drawings and in the planning statement.
Sources: 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); St Albans City and District Council (examination document, Matter 1 Issue 7.2 Appendix 1): Chilterns Beechwoods SAC Mitigation Strategy for Ashridge Commons and Woods SSSI (Part A subject to finalisation)
Neighbourhood plans
Once made, a neighbourhood plan forms part of the Development Plan for the district, and it has to be in general conformity with the strategic policies of the adopted plan. Five are in force today.
Made at Full Council on 20 February 2019, after a yes vote at referendum on 7 February 2019. Its area covers Harpenden Town and Harpenden Rural parishes.
Listed in the Development Plan as made in July 2021.
Listed in the Development Plan as made in July 2022.
Listed in the Development Plan as made in July 2023.
Listed in the Development Plan as made in July 2023.
Colney Heath and London Colney have been designated as neighbourhood plan areas, and the council's page notes that other neighbourhood plans, if made, will also form part of the Development Plan.
For a Harpenden house, the neighbourhood plan's policies sit beside the Local Plan in our brief from the outset; the Harpenden page sets them out for the town.
Sources: St Albans City and District Council: Neighbourhood Planning (web page); St Albans City and District Council: Current Local Plan and SPDs (web page)
Sign-off
Building regulations work in the district is checked by the St Albans and Watford shared building control service, which the council runs in collaboration with Watford. It is not Hertfordshire Building Control, which provides the local authority service for eight other Hertfordshire councils.
Its team gives advice on buildings and building works to protect the health, safety, welfare and convenience of the people using them. Owners are free to look elsewhere: national guidance lets you pick between your local authority's own building control team and a private registered building control approver, and an approver tells the local authority about the work by giving an initial notice.
A new or wider driveway crossing is a highways matter for Hertfordshire County Council: under the Highways Act 1980 you may not drive over a pavement or verge unless a dropped kerb has been approved and installed by the county council.
Our building regulations submission carries the engineer's calculations; the surveyor is called to inspect at each stage, and the completion certificate is passed to you at handover. Party wall notices follow their own route and go out before anything is built close to a neighbour's wall.
Sources: St Albans City and District Council: Building control (web page); St Albans City and District Council: Building Control - Shared Service; Hertfordshire Building Control: FAQs; Hertfordshire Building Control: Regulatory information; GOV.UK: Building regulations approval (printable guide); Hertfordshire County Council: Residential Dropped Kerbs Policy, version 7 (8 May 2026)
Our Hampstead base, serving St Albans District
Our studio is in Hampstead on Finchley Road, at NW3 6DN. We serve St Albans District 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 visitWorking with us
Our London team of in-house architects, designers and structural engineers, working with Hampstead Chartered Surveyors & Building Consultancy, takes on projects across St Albans and Harpenden. Planning permission is decided by St Albans City and District Council, and building regulations approval comes either from the St Albans and Watford shared building control service or, should you choose one, from a private registered building control approver.
Our Finchley Road studio in Hampstead, NW3, runs the work, and there is no separate base in Hertfordshire. We begin with a visit to the house to measure it, look at the structure and talk through what the planning position on this page allows.
From that visit come a measured survey and a feasibility drawing tested against the policies above. Once a scheme is agreed, the same people prepare the planning drawings, structural calculations and building regulations drawings, and then run the build through to sign-off, so nothing is lost between a separate designer and builder.
Outside St Albans District, 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: St Albans City and District Council: Current Local Plan and SPDs (web page); St Albans City and District Council: Building control (web page); GOV.UK: Building regulations approval (printable guide); Hampstead Renovations: Belsize Park Gardens, NW3 case study
Questions about St Albans district
The St Albans Local Plan 2026 (Parts A and B) (adopted 9 September 2026). It was adopted at an Extraordinary Council Meeting and wholly replaces the District Local Plan Review 1994; the Inspectors' Report that closed the examination reached the council on 21 August 2026. The 1994 plan, its supplementary documents and the guidance that went with it, including the 2004 Green Belt extensions guidance, are no longer part of the Development Plan and are kept online for reference only.
LG9 says extensions should not create a building that is disproportionately larger or of significantly different character than the original building, meaning the building as it stood at 1 July 1948, or as first built if later. Previous and proposed extensions are assessed together, using both volume and floor space. Table 3.3 then guides the size normally permitted by how well a scheme performs against criteria on side, front, rear, roof and basement extensions and public visibility, ranging from "Less than 90m³ increase in cubic content or Less than 20% increase in floorspace (gross)" for a scheme that performs poorly to "No more than 180m³ increase in cubic content or No more than 40% increase in floorspace (gross)" for one that performs well, in each case subject to a maximum increase in cubic content of 300m³, whichever is greater. A scheme that performs very well can go above the top of those ranges, assessed case by case.
Planning is only part of the answer. LG9 treats wholly subterranean basements with no external impact as generally acceptable, while lightwells, sunken terraces, railings and anything showing above ground are assessed like front and rear extensions. The ground then has its say: the whole district lies in source protection zones, and the 2018 flood assessment says habitable basement uses should not be permitted in Flood Zone 3, adding that basements should be avoided in high groundwater zones. A site investigation decides whether one can be built.
Usually not. The strategy screens out householder applications and replacement dwellings because they have no bearing on visitor pressure at Ashridge Commons and Woods. It screens in proposals that add dwellings within the Zone of Influence, and treats an extension or granny annexe case by case, depending on whether it functions as a separate unit to the main dwelling.
The St Albans and Watford shared building control service, rather than Hertfordshire Building Control, which serves eight other Hertfordshire councils. You may instead choose a private registered approver, and that approver informs the local authority of the work through an initial notice.
Five: Harpenden, made on 20 February 2019; Sandridge, made in July 2021; St Stephen, made in July 2022; and Redbourn and Wheathampstead, both made in July 2023. Each forms part of the Development Plan alongside the Local Plan. Colney Heath and London Colney are designated neighbourhood plan areas without a made plan so far.
See how St Albans District fits within our wider Hertfordshire pages.
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