House
extensions
in Penn, HP10
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Whether a Penn plot sits in an inset village, in a GB4 row or out in open Green Belt decides almost everything about an extension, so that is the first question we answer for any address.
Penn's houses usually stand on generous plots. The design code describes properties in Penn village with large back gardens facing the Green Belt, and Knotty Green as large houses on substantial plots, typically two to two and a half storeys high.
The Neighbourhood Plan records plot redevelopment and extensions in some Areas of Special Character in recent years while finding their essential character intact, and it notes growing pressure for infill and plot redevelopment inside the villages' inset boundaries.
Sources: Penn Parish Council / AECOM, published by Buckinghamshire Council: Penn Parish Design Guidance and Codes, final report (September 2023), Appendix A to the Penn Parish Neighbourhood Plan; Penn Parish Council / Buckinghamshire Council: Penn Parish Neighbourhood Plan 2022-2040, Made Plan (made 8 May 2025)
Inset village, GB4 row or open Green Belt
Knotty Green, Forty Green and parts of Penn are inset from the Green Belt. For those houses the 1997 plan's Policy H13 is the starting point: permission follows if neighbours suffer no significant loss of amenity, the street scene is not harmed and Policies H14 to H17 are satisfied.
Church Road and Beacon Hill, Pauls Hill and the whole of Penn Street are listed as GB4 rows, and Winchmore Hill is a GB5 settlement; extending a house there brings in Policy GB12 alongside H13 to H17. Elsewhere in the open Green Belt, GB13 lets an extension through only if it stays subordinate to the original house in size and scale and does not intrude on the landscape, and every earlier addition is added into that judgement.
National policy runs in parallel and does not treat an extension as harmful to the Green Belt unless it adds disproportionately to the original building: the one that existed on 1 July 1948 or, for anything newer, the building as first constructed. Each test is quoted in full on our Chiltern page.
The design code also warns against garden changes that damage the Green Belt boundary, giving houses with large gardens annexing the Green Belt as its example, so we confirm the lawful garden line before placing anything at the far end of a plot.
Sources: Penn Parish Council / Buckinghamshire Council: Penn Parish Neighbourhood Plan 2022-2040, Made Plan (made 8 May 2025); Chiltern District Council: Chiltern District Local Plan, adopted 1 September 1997 (alterations 29 May 2001; consolidated September 2007 and November 2011); MHCLG (GOV.UK): NPPF Chapter 13, Protecting Green Belt land (GB1-8) (August 2026); Penn Parish Council / AECOM, published by Buckinghamshire Council: Penn Parish Design Guidance and Codes, final report (September 2023), Appendix A to the Penn Parish Neighbourhood Plan
Householder rights on article 2(3) land
A house inside the Penn and Tylers Green or Penn Street conservation areas, or anywhere within the Chilterns National Landscape, stands on article 2(3) land. On that land Class A will not cover building out past a side wall, a rear addition taller than one storey, or recladding the walls in any of the materials the order lists, from render and timber to tiles.
The National Landscape does not follow village names. Natural England's map shows the Chilterns line taking in Penn Street, Forty Green and Winchmore Hill while its points for Penn and Knotty Green fall outside, so a Knotty Green house outside any conservation area may still enjoy the wider Class A rights.
The parish code itself reminds applicants that many household extensions are covered by permitted development and need no application. Where one is needed, the code counts in full, because the plan gives it the weight of the development plan.
Sources: 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); Penn Parish Council / Buckinghamshire Council: Penn Parish Neighbourhood Plan 2022-2040, Made Plan (made 8 May 2025); legislation.gov.uk: GPDO 2015 (SI 2015/596), Schedule 2 Part 1, development within the curtilage of a dwellinghouse (latest available revised); Natural England Open Data: Areas of Outstanding Natural Beauty (England), point query at the OS Open Names point for Penn Street; Natural England Open Data: Areas of Outstanding Natural Beauty (England), point query at the OS Open Names point for Forty Green (HP9); Natural England Open Data: Areas of Outstanding Natural Beauty (England), point query at the OS Open Names point for Winchmore Hill; Natural England Open Data: Areas of Outstanding Natural Beauty (England), point query at the OS Open Names point for Penn; Natural England Open Data: Areas of Outstanding Natural Beauty (England), point query at the OS Open Names point for Knotty Green; Penn Parish Council / AECOM, published by Buckinghamshire Council: Penn Parish Design Guidance and Codes, final report (September 2023), Appendix A to the Penn Parish Neighbourhood Plan
Keeping the original house in charge
Code BF 06 wants the original building to stay the dominant element however many extensions it gains, with no viewpoint from which the addition overwhelms it, no significant loss of neighbours' privacy, and no wraparound designs with overly complicated roof forms. Its height code adds that flat roofs on extensions and garages should be avoided.
The 1997 plan's H15 keeps an extension within the height of the house it joins and asks it to respect that house's scale and proportions, with materials, windows and doors that match or harmonise. It will accept a flat-roofed single-storey extension unless prominent in the street, which makes the parish code the stricter of the two. In spacious layouts, H17 wants a gap kept between a single-storey side extension and the boundary.
The conservation area appraisal accepts that old cottages often need extending for modern living, but it records damage from extensions that almost close the gaps between houses, and it expects additions not to dominate the original building and to relate to it and to its neighbours.
Materials carry real weight here. Buildings in Penn are typically red brick and flint walls under pitched roofs, the code says, and any extension should respect those materials, above all within the conservation area.
Sources: Penn Parish Council / AECOM, published by Buckinghamshire Council: Penn Parish Design Guidance and Codes, final report (September 2023), Appendix A to the Penn Parish Neighbourhood Plan; Chiltern District Council: Chiltern District Local Plan, adopted 1 September 1997 (alterations 29 May 2001; consolidated September 2007 and November 2011); Chiltern District Council and Wycombe District Council (hosted by Buckinghamshire Council): Penn and Tylers Green Conservation Area Appraisal (2008)
Party walls, building control and who decides
Building on or near a shared boundary means telling the neighbours first, and digging below and near the level of their foundations is one of the works that needs notice. Neighbours cannot stop lawful work, though they can influence its method and timing.
Our London team takes on house extensions in Penn: architects and designers who work for us directly, our own structural engineers, and Hampstead Chartered Surveyors & Building Consultancy for the survey side. Buckinghamshire Council decides the planning application. The construction is controlled by the Building Regulations 2010 and inspected by Buckinghamshire Council's building control service or, if you choose, a private registered approver.
The Neighbourhood Plan's carbon-reduction clauses expressly leave out householder applications for extensions and alterations, although the design code still asks building extensions to strengthen biodiversity and the natural environment.
Outside Penn, and in London rather than Buckinghamshire: Project example: Hampstead Garden Suburb, NW11. That project of ours added a rear extension of a single storey to an Arts and Crafts house, and had to win consent from both the Hampstead Garden Suburb Trust and the council, as its case study records.
Sources: GOV.UK: Party walls and building work - overview; GOV.UK: Party walls and building work - work you must tell your neighbour about; Buckinghamshire Council: Local Development Scheme, 3. Our current plans and documents (last updated 16 March 2026); GOV.UK: Building regulations approval - when you need approval; Buckinghamshire Council: Building regulations and control; Penn Parish Council / Buckinghamshire Council: Penn Parish Neighbourhood Plan 2022-2040, Made Plan (made 8 May 2025); Penn Parish Council / AECOM, published by Buckinghamshire Council: Penn Parish Design Guidance and Codes, final report (September 2023), Appendix A to the Penn Parish Neighbourhood Plan; Hampstead Renovations: Meadway, NW11 case study
House Extensions in Penn: Questions
Is my Penn house in the Green Belt, and which extension test will the council apply?
Probably, unless it stands in Knotty Green, Forty Green or the inset part of Penn: the plan describes the parish as washed over, with 80% of its area Green Belt and National Landscape. Inset houses are judged under H13 to H17, the GB4 rows and the Winchmore Hill GB5 area under GB12 with the same criteria, and other Green Belt houses under GB13, which wants extensions kept subordinate to the original house and out of the landscape's way.
Do I need planning permission to extend a Penn house inside the National Landscape?
Not for everything, but National Landscape land counts as article 2(3) land, so building out to the side, adding more than one storey at the rear or recladding the walls falls outside permitted development there. Natural England's map includes Penn Street, Forty Green and Winchmore Hill but not its Penn and Knotty Green points, and national policy wants development in these landscapes limited in scale and sensitively located and designed.
How does Penn's Neighbourhood Plan treat house extensions?
Policy PENN1 makes its design code part of the plan, and the code's extension rule keeps the original building dominant, rules out additions that overwhelm it from any viewpoint, protects neighbours' privacy and discourages wraparound designs with complicated roofs. The plan's carbon clauses do not apply to householder extensions.
When does a Penn extension involve the Party Wall Act?
Notice is needed for building on or at the boundary line, for cutting into a party wall, and for excavating close to a neighbour's building, the distance being 3 or 6 metres depending on how deep the foundations go. Because most Penn houses are detached, it usually comes down to foundations near the boundary rather than a shared wall, and a dispute is settled by a surveyor or surveyors under the Act.
Who inspects and certifies a Penn extension under the building regulations?
Buckinghamshire Council's building control service, or a private registered building control approver you appoint instead, who notifies the council with an initial notice. The 2010 regulations govern how an extension is built, and without approval you would lack the certificates of compliance a future sale may need.
What should I prepare before you look at extending my Penn house?
Find any earlier planning decisions, deed plans and drawings of past extensions, because GB13 counts earlier additions cumulatively. Clear a way along the side and rear walls and the boundaries so we can see the foundations, drains and any trees close to where the extension would go.
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