Barnet Extension Guide 2026: Planning Permission, Costs & Local Rules
London's most populated borough has a varied housing stock and a planning landscape that ranges from straightforward permitted development to some of the most restricted land in London. Here is what you need to know before extending in Barnet.
Quick Answer
Most of Barnet has standard permitted development rights — permitted development applies across the majority of the borough outside conservation areas. Key exceptions: Hampstead Garden Suburb (no PD rights, Trust approval required) and Totteridge Green Belt. Costs run slightly below inner London: rear extensions from £55k–£85k inc VAT.
£55k–£85k
Rear extension (3m×5m)
£55k–£90k
Loft conversion
£100k–£160k
Two-storey rear
Check your specific property constraints
Free Property CheckBarnet: London's Largest Borough
With around 400,000 residents, Barnet is the most populated London borough. It covers a large and varied area — from the outer edges of Zone 2 at Finchley Central to the leafy fringes of the M25 at Hadley Wood. That geographic spread means housing stock and planning character vary considerably from one postcode to the next.
The borough has one of the highest homeownership rates in outer London, which translates directly into high extension demand. Families who bought into Barnet's large Victorian and Edwardian semis tend to extend rather than move when they need more space, and the numbers generally support that decision.
Housing stock by area
Large Victorian and Edwardian semis with generous rear gardens. Good depth available for rear extensions. Many properties have already been extended once, so the "original rear wall" question requires care.
Predominantly 1930s semis. Typical rear gardens of 12–18m. These properties suit 3–4m rear extensions well and often have side returns worth infilling. Standard permitted development rights apply in most of these areas.
Larger detached properties. Totteridge sits partly in the Green Belt. Hadley Wood contains some of the borough's most expensive properties. Both areas require careful planning assessment before committing to a scheme.
A distinct and highly controlled area straddling the Barnet/Camden border. Governed by the Hampstead Garden Suburb Trust in addition to the council. No permitted development rights apply here at all. See the dedicated section below.
Barnet Local Plan and Extension Policy
Barnet adopted its Local Plan in 2012 with updates since. For residential extensions, the key policy is DM01 (Protecting Residential Amenity). DM01 sets out the council's expectations on impact to neighbours, including daylight, sunlight, overlooking, and the visual bulk of extensions as seen from adjacent properties.
The council also publishes a Residential Extensions and Alterations Supplementary Planning Document (SPD), which gives detailed guidance on acceptable depths, heights, set-backs, and materials for different extension types. Any application for planning permission in Barnet will be assessed against this SPD.
What the SPD covers
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Hampstead Garden Suburb: The Most Restricted Area in Barnet
Hampstead Garden Suburb (HGS) is one of the most carefully controlled residential areas in London. Built from 1907, it was designed as a model garden suburb by Raymond Unwin and Barry Parker, and its character has been protected ever since by the Hampstead Garden Suburb Trust.
The key point for homeowners: there are no permitted development rights in Hampstead Garden Suburb. An Article 4 Direction removes PD rights across the entire estate, meaning virtually any physical change to the exterior of a property — including extensions, roof alterations, and new windows — requires planning permission from the London Borough of Barnet.
In addition to LBB planning permission, most works in HGS also require approval from the Hampstead Garden Suburb Trust. The Trust operates under a private Act of Parliament and assesses proposals against its own design standards, which are more demanding than the council's.
What this means in practice
- No rear or side extension can be built under permitted development — all require full planning permission
- Trust approval is separate from — and in addition to — LBB planning permission. Both must be obtained
- Materials, roof pitch, window proportions, and garden boundary treatments are all subject to Trust scrutiny
- Architect fees and project timelines are higher than standard: budget an additional 20–30% over equivalent projects elsewhere in Barnet
Pre-application meetings with both the Trust and the council are strongly recommended before commissioning drawings. The Trust charges a fee for pre-application advice and formal submissions.
Totteridge: Green Belt Controls
Parts of Totteridge fall within the Metropolitan Green Belt. Green Belt designation does not automatically prohibit extensions, but it adds a layer of policy consideration. Extensions to existing dwellings are generally acceptable in the Green Belt provided they are not disproportionate in size relative to the original building.
Barnet's planning policy applies a cumulative limit: extensions to Green Belt dwellings must not increase the original floor area by more than 30% in total (including any previous extensions since 1948). This applies whether or not individual extensions required planning permission.
If you are considering extending a Totteridge property that has already been extended, commission a measured survey and calculate the cumulative extension floor area before instructing an architect. A scheme that exceeds the 30% threshold will face a very difficult planning route.
Conservation Areas in Barnet
Barnet has a number of designated conservation areas. The main ones affecting residential extensions are:
Hampstead Garden Suburb
Designated conservation area and Article 4 direction in force. As covered above, all works require LBB planning permission and Trust approval. The most demanding conservation regime in the borough.
Monken Hadley
A historic village near Cockfosters with a distinct character. Extensions visible from the street or affecting the roofline require full planning permission. Materials must respect the local vernacular — typically stock brick and clay tile.
East Barnet Village
Covers the historic village centre and surrounding residential streets. Standard conservation area restrictions apply: planning permission required for rear extensions in the conservation area; materials and design subject to greater scrutiny.
Friern Barnet Town Centre and New Barnet
Smaller conservation areas covering town centre streets and adjacent residential. Extensions to houses within these boundaries require planning permission. Outside the immediate conservation area boundary, standard PD rights typically apply.
In all conservation areas in Barnet, permitted development rights for extensions are removed or curtailed. You will need full planning permission for any extension, and the council will assess whether the proposal preserves or enhances the character and appearance of the conservation area.
Not sure if your Barnet property is in a conservation area or affected by Article 4? Our AI checks your address instantly.
Check now — it's free →Permitted Development in Barnet
For most properties in Barnet — outside conservation areas, Hampstead Garden Suburb, and Green Belt parcels — national permitted development rights apply in full. This is one of the more permissive situations among London boroughs: Barnet has not blanket-applied Article 4 directions to large residential areas in the way that Camden or Islington have.
For a standard semi-detached or terraced house in, say, Finchley, North Finchley, Whetstone, Edgware, or Colindale, the national rules apply:
| Extension type | PD limit | Prior approval limit |
|---|---|---|
| Single-storey rear (terraced/semi) | 3m depth | Up to 6m |
| Single-storey rear (detached) | 4m depth | Up to 8m |
| Single-storey rear (height) | 4m max (3m eaves within 2m of boundary) | Same height limits apply |
| Side extension | Half width of original house | N/A — full planning if over PD limit |
| Two-storey rear | 3m depth, 7m from rear boundary | N/A — almost always full planning in practice |
Before relying on PD rights, confirm your property is not in a conservation area and check whether any Article 4 direction applies. The easiest way to do this is via Barnet's planning portal or our free AI checker, which looks up your address against the relevant designations.
Planning Applications in Barnet
When full planning permission is required, applications are submitted through Barnet's planning portal (the council uses the national Planning Portal). The standard statutory decision period is 8 weeks for householder applications.
Application process
Extension Costs in Barnet 2026
Barnet sits in outer north London. Build costs run slightly below inner London pricing — roughly 5–10% below zones 1–2 benchmarks for the same specification. All figures below include VAT at 20% and cover construction cost only (not kitchen, architect fees, or contingency).
| Extension type | Typical size | Construction cost (inc VAT) | All-in estimate |
|---|---|---|---|
| Rear extension (single-storey) | 3m × 5m (15m²) | £46k–£70k | £55k–£85k |
| Side extension (infill) | 2m × 8m (16m²) | £42k–£65k | £50k–£80k |
| Loft conversion (dormer) | 25–35m² | £46k–£75k | £55k–£90k |
| Two-storey rear extension | 3m × 5m × 2 (30m²) | £84k–£132k | £100k–£160k |
| Wrap-around (rear + side) | 25–35m² | £68k–£110k | £82k–£132k |
Conservation area / HGS premium: Add 15–25% for works requiring matching heritage materials, specialist brickwork, and additional planning/Trust fees.
Professional fees to add
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Planning Tips for Barnet
Check Article 4 before assuming PD applies
Barnet's Article 4 directions are concentrated in conservation areas and Hampstead Garden Suburb, not spread borough-wide. But a property on the edge of a conservation area may be inside the boundary without the owner realising. Always verify before starting any work under PD.
Hampstead Garden Suburb: approach Trust and council in parallel
Do not submit to the council without first obtaining feedback from the Trust. The Trust meets on a set schedule; missing a meeting cycle can add 6–8 weeks to your programme. Commission an architect experienced in HGS work — they will know the Trust's current priorities and design preferences.
Pre-application advice is worth the fee in sensitive areas
For any extension in a conservation area, near the Green Belt boundary, or involving a listed building, Barnet's pre-application service gives you officer feedback before you commit to a full design. The cost (£250–£350 for householder pre-app) is a fraction of the fee for amending a refused application.
Totteridge: calculate cumulative extension before commissioning drawings
Green Belt dwellings in Totteridge are subject to a 30% cumulative floor area limit. Get a measured survey of the existing property, including any extensions already built, and confirm how much additional floor area remains within policy before your architect starts designing.
Match materials to the host building — especially in conservation areas
Barnet's Residential Extensions SPD is explicit about materials. In conservation areas, matching brick colour, texture, and mortar colour is a material consideration. For Victorian and Edwardian properties, source London stock brick samples early — reclaimed stock can be hard to match exactly and lead times can be long.
Frequently Asked Questions
Do I need planning permission for a house extension in Barnet?
In most parts of Barnet, permitted development rights apply and a single-storey rear extension up to 3m (terraced/semi) or 4m (detached) does not need planning permission. Conservation areas, Hampstead Garden Suburb, and some Green Belt areas are exceptions where full planning permission is always required. Check your specific address before assuming PD applies.
What are the rules for extending in Hampstead Garden Suburb?
Hampstead Garden Suburb is one of the most restricted areas in London. There are no permitted development rights: all extensions require full planning permission from the London Borough of Barnet. In addition, most external works also require approval from the Hampstead Garden Suburb Trust, which operates under its own Act of Parliament. You need both approvals before building.
How much does a house extension cost in Barnet in 2026?
All figures inc VAT: a 3m×5m single-storey rear extension costs £55k–£85k all-in (construction plus architect, structural engineer, building regs, and contingency). A side extension runs £50k–£80k. A loft conversion costs £55k–£90k. A two-storey rear extension costs £100k–£160k. Add 15–25% for conservation area or Hampstead Garden Suburb works requiring heritage materials and specialist design.
What conservation areas are there in Barnet?
Barnet has several conservation areas including Hampstead Garden Suburb (with its own Trust governance), Monken Hadley, East Barnet Village, Friern Barnet Town Centre, and New Barnet. In all conservation areas, extensions require full planning permission and must preserve or enhance the character and appearance of the area. Materials and design are subject to greater scrutiny.
Does Totteridge have Green Belt restrictions on extensions?
Parts of Totteridge are in the Metropolitan Green Belt. Extensions are generally acceptable provided they are not disproportionate to the original building. Barnet applies a cumulative limit: extensions to Green Belt dwellings must not increase the original floor area by more than 30% in total (including all previous extensions since 1948). Calculate your cumulative extension area before starting any design work.
How long does planning permission take in Barnet?
Standard householder applications target 8 weeks from validation. Conservation area and Hampstead Garden Suburb applications typically take 10–14 weeks. Pre-application advice is available for £250–£350 (householder) and is recommended in sensitive areas. If the Hampstead Garden Suburb Trust is involved, allow additional time for Trust meeting cycles.
What is the Barnet Residential Extensions SPD?
The Residential Extensions and Alterations Supplementary Planning Document is Barnet council's detailed guidance on acceptable extension design. It covers rear extension depths and heights, side extension set-backs (minimum 1m from the front elevation), roof alterations, and materials requirements. Any planning application for an extension in Barnet is assessed against this SPD.
Do I need a party wall agreement for an extension in Barnet?
Almost certainly yes for a terraced or semi-detached house. The Party Wall Act 1996 applies whenever you excavate foundations within 3m of a neighbouring structure — which covers the vast majority of extensions in Barnet. Serve notice at least 2 months before work starts. Budget £800–£1,400 per adjoining neighbour for surveyor fees.
Summary
For most Barnet homeowners — in Finchley, Edgware, High Barnet, Whetstone, New Barnet, Colindale, or Mill Hill — the planning landscape is relatively straightforward. Permitted development rights apply in full, the council's SPD sets clear design parameters, and costs run slightly below inner London.
The major exceptions are Hampstead Garden Suburb (no PD rights, requires Trust approval in addition to planning permission) and Green Belt parts of Totteridge (cumulative floor area limit applies). If your property falls into either category, engage an architect with specific experience of those regimes before commissioning a scheme.
The first step for any Barnet extension project is confirming your planning status: conservation area, Article 4, Green Belt, or listed building. That single question determines your route and your realistic timeline.
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