Area guide · HA7
House Extensions in Stanmore & Bushey Heath (HA7)
Planning a house extension in Stanmore or Bushey Heath? The wooded ridge between them is one of north-west London's quiet wealth pockets: substantial detached houses on tree-screened plots climbing from Stanmore towards the common, the 1930s modernist run of Kerry Avenue, and the Brockley Hill corridor where the houses turn into small estates. It is also a borough boundary: Stanmore answers to Harrow, Bushey Heath to Hertsmere, and which desk your application lands on changes the policies it is judged against. The briefs up here are big, double-storey reworks, basements and whole-house programmes, and reading the boundary correctly is step one of winning them.
Planning in Stanmore & Bushey Heath
What the two councils approve on the ridge
The boundary runs along the top of the hill, so near-identical houses a few hundred metres apart can answer to different rulebooks. Harrow decides the Stanmore side, Hertsmere the Bushey Heath side, and both regularly approve the staple householder schemes, rear extensions and lofts among them, when the application is properly prepared.
What the ridge adds is designation reading. Parts of Stanmore carry conservation areas, the upper slopes meet green belt, and the tree cover is protected enough that siting often decides a scheme before style does. None of it prevents the big projects; all of it rewards the design-first preparation we bring to every luxury extensions job.
Sources: the published planning registers of Harrow and Hertsmere. Register patterns describe the approvals weather; every application turns on its own design.
Local knowledge
The ridge's stock, road by road
The avenue detached belt. Along Gordon Avenue, Wood Lane and the roads under the hill, substantial inter-war detached houses on wide plots take the classic premium schemes: wide glazed rears (£55,000 to £120,000), wrap-arounds (£95,000 to £180,000) and double-storey reworks (£100,000 to £215,000) that read as original.
Kerry Avenue and the modernist pocket. Stanmore's 1930s modernist houses are a design responsibility: flat roofs, white render, curved glass. Extending them demands their own language rather than a brick pastiche, and done properly the results are spectacular.
The common and the green edges. Towards Stanmore Common and Warren Lane, plots meet woodland and the green belt frame appears. Additions here are sited around root protection and long views, with the proportion arguments from our green belt guide doing the policy work.
Brockley Hill and Bushey Heath. The corridor towards Elstree is the top of this market: gated houses on deep plots where the brief is usually extension plus whole-house remodel, and where Hertsmere is the authority to design for.
Every street is different. That's the point.
We design to Stanmore & Bushey Heath's stock and to what the council demonstrably approves — not to a template. A free site visit tells you what your house can take.
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What an extension costs here in 2026
All-in build ranges, excluding VAT, professional fees and kitchen fit-out:
| Extension type | Typical cost | On-site time | Usual planning route |
|---|---|---|---|
| Side-return extension | £45,000 to £85,000 | 2 to 3 months | Often permitted development |
| Single-storey rear | £55,000 to £120,000 | 3 to 4 months | Often permitted development |
| Kitchen extension (incl. mid-range kitchen) | £70,000 to £140,000 | 3 to 4 months | Often permitted development |
| Hip-to-gable loft + rear dormer | £65,000 to £115,000 | 6 to 10 weeks | Usually permitted development |
| Wrap-around extension | £95,000 to £180,000 | 4 to 6 months | Full application |
| Double-storey rework | £100,000 to £215,000 | 5 to 7 months | Full application |
Budget £3,300 to £5,300 per square metre depending on side of the hill and specification. The larger bands are set out in the luxury extension cost guide, and the quote is always one fixed, itemised figure after a free visit.
Permissions, handled
Planning on the boundary: what to know
Confirm the authority first. Harrow and Hertsmere differ in policy detail and process. We establish yours at the outset and prepare the file for that desk.
Permitted development mostly survives. Outside the conservation pockets, detached houses keep the 4m and prior-approval 8m rear allowances plus loft volume, and we certificate everything built that way with a Lawful Development Certificate.
Trees before floor plans. Protected woodland cover shapes siting on the upper roads. Root protection surveys come early in our process because they decide what the plot can actually take.
Party walls. Boundary excavation, basements included, triggers the Act as usual: budget £1,000 to £2,500 per affected neighbour where surveyors are appointed, notices managed by us.
How it works
How it works
- Free design visit. Authority confirmed, designations and trees mapped, honest advice on scope and budget.
- Design and drawings. Composed to the house's own language, modernist or traditional, and the policies it must satisfy.
- Planning, handled. The application managed through Harrow or Hertsmere to the decision notice.
- Fixed-price build. One signed figure, delivered by the team we trust with our name, handed over immaculate.
Questions, answered
Stanmore & Bushey Heath extension FAQs
Which council will decide my application?
Harrow for Stanmore addresses and Hertsmere for the Bushey Heath side. The boundary runs along the ridge, so we confirm your exact position first; it changes the policies, the forms and occasionally the tactics.
What does an extension cost up here?
Plan on £3,300 to £5,300 per square metre in 2026. Glazed rears typically run £55,000 to £120,000 and double-storey reworks £100,000 to £215,000, excluding VAT and fees, with the whole-house programmes banded in our luxury cost guide. Every quote is fixed and itemised.
Can you extend the modernist houses sympathetically?
Yes. The modernist houses suit flat-roofed, rendered additions with crittall-style glazing that extend the original architecture rather than argue with it. Conservation designations on those streets make the design case decisive, which suits a design-led application.
Does the green belt affect me?
Only on the upper plots that meet it, where extensions are measured against the original building. We calculate that headroom at the first visit and design inside it.
Is a basement realistic on the ridge?
On the larger plots, often yes, subject to ground conditions we assess early. It adds a leisure floor without touching the tree screening or the street presence, priced on design as a full-application project.
How long will it all take?
A few weeks of design, roughly eight in planning, then three to seven months on site by scale. Programme dates come with the quote and we hold ourselves to them.
What we build in Stanmore & Bushey Heath
The same team designs, gets planning and builds — one fixed, itemised price whichever route your Stanmore & Bushey Heath project takes.
- Kitchen extensions in Stanmore & Bushey Heath
- Side return extensions in Stanmore & Bushey Heath
- Double-storey & wrap-around in Stanmore & Bushey Heath
- Loft conversions in Stanmore & Bushey Heath
- Basement conversions in Stanmore & Bushey Heath
- Whole-house transformations in Stanmore & Bushey Heath
Checking the rules for Stanmore & Bushey Heath first?
What clients say
Rated 5.0 on Google
“The Extension Company built a 5m single storey rear extension on our Victorian terrace in Crouch End. Went down the permitted development route so no planning application needed, which they explained up front and saved us months. Kitchen-diner now opens onto the garden with sliding doors. £87k all in, matched the quote.”
“From the initial site visit onwards to my property in Barnet, we felt that the team actually listened to what we wanted. They suggested a few changes to the layout that we hadn't considered, and those changes have made a huge difference. Would definitely use again and recommend”
“Used The Extension Company for a loft conversion on our semi detached in Barnet and couldn't be happier. They explained what was possible at the first visit, sorted the planning side, and gave a fixed price with no surprises at the end. The hip-to-gable and rear dormer gave us a big double bedroom with an en-suite. Tidy, professional and finished on schedule. Would recommend to anyone in North London thinking about a loft conversion.”
“We had been putting off our loft conversion for years because the whole process felt overwhelming. The team made it much easier than expected and handled the drawings, planning and build for us. We now have a lovely extra bedroom and shower room.”
“Our wrap-around extension has completely transformed the ground floor. We now have the open plan kitchen and family space we wanted without losing all of the garden. Thanks to everyone involved.”
“Really pleased with the work from The Extension Company. We had our extension done in Hampstead and the whole experience was excellent. Great communication from start to finish, The quality of the finish is excellent and we’re really happy with the result. Would definitely recommend them if you’re thinking about an extension.”
Reviews shown word for word from our Google profile. See 9 more client reviews · Read all 16 reviews on Google
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