Hampstead Architecture and Planning

Barnet

House Extension Architects in Hendon

Hendon is mostly interwar semi-detached housing on generous plots, and that combination changes the starting question. In much of inner north west London the first task is establishing what a conservation designation prevents. Here, for most addresses, the more useful question is how much you can build without needing planning permission at all — and the answer is usually more than owners expect.

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The short answer

How much can you extend a Hendon house without permission?

On an undesignated interwar semi, often a great deal. Class A permitted development covers a substantial single-storey rear extension on a semi-detached or detached house, and the prior approval procedure allows a materially deeper one subject to neighbour notification. Two checks decide it: whether the address falls within one of Barnet’s conservation areas, and whether a previous owner’s extension has already used part of the allowance, which is measured against the original house.

Planning at a glance

Hendon

Planning at a glance in Hendon
Local planning authorityBarnet
Likely approval routeHouseholder planning application or permitted development, depending on conservation status, listing and property type — confirmed for the exact address before drawings begin
Statutory determination time8 weeks for most householder applications (13 weeks for major applications)
Typical drawing package£2,250–£3,250 inc. VAT, fixed fee. Find my package →

The interwar semi is the dominant house type and it responds well to extension because of how it was planned. These houses were built with a front reception room, a rear reception room, a small kitchen at the back and often a side passage or garage to the flank, on plots with real depth. That means there are usually two directions to work in rather than one: rearwards into a long garden, and sideways into a gap that a Victorian terrace simply does not have. The best schemes on these plots use both, because widening and deepening together allows the whole ground floor to be reorganised rather than merely enlarged.

Because most of Hendon is undesignated, the permitted development route is genuinely available, and it is worth understanding what it permits before commissioning a full planning application by default. Class A allows a single-storey rear extension on a semi-detached or detached house up to a set depth with a height limit and eaves limit; the prior approval procedure allows a materially deeper single-storey rear extension in exchange for a neighbour notification period, during which an objection triggers the council to assess the impact. Together those two routes cover a large proportion of what people actually want to build, without a full application and its determination period.

A full householder application is still worth making where the ambition exceeds those envelopes — a two-storey side or rear extension, a side extension wider than the Class A limit, or a scheme where the roof form matters. On a street of matched semis, the assessment of a two-storey side extension turns on whether it preserves a sense of separation between pairs, so setting the upper floor back from the front elevation and keeping a visible gap at the boundary is what makes it acceptable rather than a stylistic preference.

Two practical matters shape the build. First, ground conditions: much of this area sits on clay, which requires deeper foundations and makes nearby trees a genuine design consideration, both existing ones and any removed in recent years, because ground can heave for a long time after a large tree goes. Second, the arterial roads: houses fronting the main routes through Hendon have real noise exposure, and an extension that opens up the back of the house is an opportunity to improve acoustic performance at the front at the same time, which is far cheaper done together.

Local analysis

What matters in Hendon

Using permitted development properly on a Hendon semi

On an undesignated semi-detached house the permitted development route is not a compromise; it is frequently the sensible way to build. Class A sets an envelope — a depth limit for a single-storey rear extension, a maximum height, an eaves height limit where the extension is within two metres of a boundary, restrictions on side extensions to no more than half the width of the original house and single storey, and a requirement that materials be of similar appearance. Within that envelope no planning application is needed. Beyond it, the prior approval procedure allows a substantially deeper single-storey rear extension: you notify the council, adjoining neighbours are consulted, and if none objects the work can proceed, while an objection triggers an assessment of the impact on amenity rather than a full policy determination. Two things are worth doing even when no permission is required. First, obtain a lawful development certificate: it is the document a future buyer’s conveyancer will ask for, and it is far easier to get while the works are recent. Second, check the "original house" definition, because the allowances are measured against the house as first built or as it stood in 1948, not as you bought it — a previous owner’s extension eats into your allowance and is the single commonest reason a permitted development scheme turns out not to be permitted.

Clay, trees and foundation depth

Foundations are the largest variable in the cost of a Hendon extension and the depth is not knowable by looking. The clay across much of this area shrinks and swells with moisture content, so foundations must reach a depth where seasonal variation has little effect, and that depth increases substantially in the vicinity of trees, which draw moisture from the soil across a radius related to their species and mature height. The considerations are the species, the mature height, the distance from the proposed foundation, and — often overlooked — whether any large tree has been removed within recent years, because ground that has been dried by a tree for decades can heave for a long time afterwards, which is a different and sometimes more damaging problem than shrinkage. On the wider Hendon plots there are frequently mature trees in the garden or on the neighbouring boundary, and any tree that is protected by a preservation order or stands within a conservation area brings a separate consent requirement before it can be pruned or felled. A site investigation at feasibility informs the foundation design, tells you whether a reinforced raft or a piled solution is likely, and converts the biggest open contingency in the budget into a figure you can plan around.

Thinking about a project in Hendon?

Send the address and a short description. We will confirm the likely approval route and the drawings your project needs.

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Local planning

Planning & conservation context

Barnet is the planning authority at the checked Hendon research point and throughout the surrounding radius, so there is no boundary question to resolve here. That makes recent Barnet decisions on similar houses genuinely comparable evidence rather than a false analogy.

House extensions run through Schedule 2, Part 1, Class A of the General Permitted Development Order. The allowances are larger for detached and semi-detached houses than for terraces, which is why a substantial single-storey rear extension is often available on a Hendon semi without any planning application at all.

Barnet has ten conservation areas that are either covered by, or have, Article 4 directions in force. Where an address falls within one, named permitted development rights are withdrawn and a householder planning application is required, so the designation check comes before any allowance is assumed.

The conservation records within 1,400 metres of the Hendon point are all Barnet designations: Hendon Church End, Hendon The Burroughs, College Farm and Finchley Garden Village. Each has its own boundary and covers a limited part of the wider Hendon area.

Hendon Church End is a linear designation focused on St Mary’s Parish Church at the top of Greyhound Hill — the high ground that gives Hendon its name, from an old term meaning at the high down. Hendon The Burroughs is characterised largely by terraced residential development with some offices and shops along a busy route from Watford Way.

Most Hendon housing sits outside all four. That does not make the check optional — it makes it quick and decisive, because a negative result opens the permitted development and prior approval routes that a designation would close.

For the wider local-authority picture — policies, timescales and application routes — see planning in Barnet.

Illustrative study — street elevation

Services

Architectural services in Hendon

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Nearby

Nearby House Extension Architects pages

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Architecture in Hendon: FAQs

Do I need planning permission to extend a house in Hendon?

Often not. Most Hendon housing is undesignated interwar semi-detached and detached stock, where Class A permitted development allows a substantial single-storey rear extension and the prior approval procedure allows a deeper one. Where the address falls within one of Barnet’s conservation areas — ten of which are covered by or have Article 4 directions in force — those rights are withdrawn and a householder application is required.

Which council covers Hendon?

Barnet, throughout. The checked Hendon research point and the surrounding 1,400-metre radius return Barnet and no other authority, which removes the boundary question that dominates projects a few miles south. The practical benefit is that recent Barnet decisions on similar houses nearby are genuinely comparable evidence, because they were determined under the same local plan and the same guidance.

What is the prior approval route and should I use it?

It allows a single-storey rear extension deeper than the standard Class A limit on an undesignated house. You notify the council, adjoining neighbours are consulted, and if none objects the work proceeds; an objection triggers an assessment of the impact on neighbour amenity. It is faster than a full application and it covers a lot of what people actually want to build, so it is usually worth testing before committing to a householder application.

Does a previous owner’s extension use up my allowance?

Yes, and it is the commonest reason a permitted development scheme turns out not to be permitted. The allowances are measured against the original house — as first built, or as it stood in 1948 — rather than as you bought it. A rear addition or a side extension added by a previous owner counts against your remaining entitlement. Establishing what is original is part of the survey rather than an assumption.

Should I get a lawful development certificate?

Yes, whenever you build under permitted development. It is the council’s formal confirmation that the work did not require planning permission, and it is the document a future buyer’s conveyancer will ask to see. Obtaining it while the works are recent, the drawings are to hand and the evidence is clear is far easier than trying to reconstruct the position years later when the house is under offer.

How deep will the foundations need to be?

Deeper than on gravel, because much of this area sits on clay which shrinks and swells with moisture content. The depth increases substantially near trees, and ground where a large tree was removed in the last few years can still be heaving. A site investigation at feasibility informs the design, indicates whether a raft or piles are likely, and turns the biggest variable in the budget into a known figure.

Can I build a two-storey side extension on my semi?

Often, though it needs a full householder application since two-storey side extensions fall outside Class A. The assessment turns on whether the addition preserves a sense of separation between the semi-detached pair and along the street, so setting the upper floor back from the front elevation and keeping a visible gap to the boundary is what makes it acceptable. Barnet decisions on comparable houses nearby are useful supporting evidence.

What does a house extension architect in Hendon charge?

We publish fixed-fee packages rather than quoting a percentage before seeing the property, starting with a route review credited against the design work if you proceed. That review tells you which of permitted development, prior approval or a full application applies. Later stages cover the consent drawings, then building regulations and tender information. Structural engineering, site investigation and council fees sit outside the architectural fee.

How much does a house extension cost in Hendon?

Fees for house extensions in Hendon follow scope, consent complexity and whether freeholder or specialist input is required. Published packages for related work start from around £2,250–£3,250 once scope is clear; council, engineer and freeholder surveyor fees are usually separate. See our packages page for current fixed-fee bands — we confirm a fee only after the address and proposed work are understood.

Discuss a house extension in Hendon

Send your Hendon address, photographs of the rear elevation and the side of the house, and details of any extension a previous owner built. We will confirm the designation position, work out what remains of your permitted development allowance, and set out whether prior approval or a full application is the right route.

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