TL;DR — Key Facts
- Build cost per sqm: £2,200 – £3,200 (outer London borough)
- Council planning fee: £548 (householder application, 2026)
- Typical projects: rear extension, side-return, two-storey, wraparound
- Conservation areas: 20 designated areas in Barnet
- Permitted Development: Generally available outside conservation areas
What does Barnet do with an extension application?
It approves close to the London average and loses more appeals on appeal than all but one other borough. Barnet approved 83.5% of 4,584 householder decisions, and the Inspectorate allowed 49.1% of the 927 appeals that followed a refusal. Barnet also holds 3,215 tree preservation zones, the second highest count in London.
| Measure | Barnet | London |
|---|---|---|
| Householder approval rate | 83.5% | 83.0% |
| Householder decisions made | 4,584 | 77,680 |
| Householder appeals allowed after refusal | 49.1% of 927 | 40.8% of 13,067 |
| Tree preservation zones in the register | 3,215 | second most of the 9 boroughs in the tree register |
| Enforcement notices issued | 309 | 3,554 |
| Householder planning application fee | £548 | statutory, England 2026 |
Sources: decision and enforcement figures from MHCLG District Planning Application Statistics (PS1 and PS2), twelve quarters to March 2026; the tree register from the MHCLG Planning Data Platform, retrieved 1 September 2026; appeals from Planning Inspectorate casework decided 2016–2026; the fee from the Fees for Applications Regulations 2012 as amended. Build costs on this page are our own 2026 London range.
What does a Barnet extension cost in 2026?
Once the budget works, the next step is the drawing set: house extensions in Barnet.
House extension build costs in Barnet typically range from £2,200 to £3,200 per square metre, reflecting Barnet's position as an outer London borough. These figures cover the construction cost only and do not include professional fees, planning fees, or VAT.
On a Barnet semi the swing factor is the width of the opening you want at the back. A modest set of doors sits at the bottom of the range; a full-width opening across a 1930s rear elevation needs a substantial steel, and on a semi that beam usually bears into the party wall, which brings its own design and notice work. Specification is secondary to span here.
Typical extension costs in Barnet (build only)
On top of the build cost, you will need architectural drawings. These fees are 30% below typical London architect rates.
What moves the price of a Barnet extension?
Barnet's interwar semis mostly sit on plots with a side passage, which keeps groundworks straightforward and is the single largest cost variable on a rear extension. The borough is also the heaviest user of the certificate route in London, issuing 2,585 lawful development certificates in three years (MHCLG planning statistics, retrieved 1 September 2026). Where the property is in Hampstead Garden Suburb, the Suburb's own Scheme of Management adds a consent and a timetable the rest of the borough does not have.
Conservation areas
Barnet has 20 designated conservation areas, including Hampstead Garden Suburb, Mill Hill, Hendon, Golders Green, Monken Hadley. If your property is in a conservation area, Permitted Development rights are restricted, meaning you may need a full planning application (£548 council fee). Conservation area applications also tend to require higher-quality materials and more sympathetic design, which can increase build costs by 10-20%.
Property type
The housing stock in Barnet is predominantly 1930s semi-detached dominates; edwardian villas in muswell hill, finchley; garden suburb arts & crafts in hampstead garden suburb; post-war flats in colindale. Victorian and Edwardian terraces typically suit side-return and rear extensions, while 1930s semi-detached properties are well suited to two-storey rear extensions. Detached houses offer the most flexibility but often have larger footprints, meaning higher absolute costs even if the per-sqm rate is similar.
Access and logistics
A 1930s Barnet semi almost always has a side return wide enough to get a mini digger and a wheelbarrow to the rear, which is why extensions here price more predictably than in inner London. The Edwardian villas of Muswell Hill and Church End are often terraced or tightly spaced and do not have that. Establish the access route before the quote, not after: it decides how the groundworks are priced.
Our drawing fees for Barnet extensions
At Architectural Drawings London. No hourly billing, no surprises. Our fees cover everything needed for a valid planning submission to Barnet Council and for building regulations approval.
Drawing fees for extensions in Barnet
Every package includes a measured survey of your existing property, existing floor plans and elevations, proposed floor plans and elevations, a site plan, and a design and access statement where required. Our MCIAT chartered architectural technologists handle the full process from survey to submission, and we liaise with Barnet Council on your behalf.
We cover all Barnet postcodes: N2, N3, N10, N11, N12, N14, N20, NW2, NW4, NW7, NW9, NW11, EN4, EN5.
Planning or permitted development in Barnet?
Many house extensions in Barnet can be built under Permitted Development (PD) rights without needing a planning application. Under PD, single-storey rear extensions can extend up to 6 metres from the original rear wall for terraced and semi-detached houses, or 8 metres for detached houses (under the Prior Approval process).
Across Barnet's 20 conservation areas the loss that bites is the side extension. This is a borough of interwar semis with side passages, and that passage is precisely where an unrestricted owner would build; inside a designated area they cannot, under permitted development. Rear depth is also cut, and any cladding or render change needs an application.
Two-storey extensions require an application in nearly every case. The question that decides them in Barnet is what the addition does to the semi-detached pair: these houses were built as matched halves, and a first floor carried out to the shared boundary reads as one half of the pair growing away from the other. Keeping the upper storey inset and below the main ridge is what usually resolves it.
With 2,585 certificates issued in three years, more than any other London borough (MHCLG planning statistics, retrieved 1 September 2026), Barnet is a place where the certificate is the normal route rather than a precaution. Take one even where the position looks obvious. It is the document that ends the conversation at sale, and it costs a small fraction of a full application.
Frequently asked questions
How much does a house extension cost in Barnet?
Build costs for a house extension in Barnet typically range from £2,200 to £3,200 per square metre, as Barnet is an outer London borough. A typical single-storey rear extension of 15-20 sqm would cost between £33,000 and £64,000 for the build alone. On top of build costs.
Can I extend under Permitted Development in Barnet?
Many single-storey rear extensions in Barnet qualify as Permitted Development, allowing you to build without planning permission. You can extend up to 6 metres from the rear wall for terraced and semi-detached houses, or up to 8 metres for detached houses under Prior Approval. However, properties in Barnet's 20 conservation areas have more restricted PD rights. We recommend applying for a Lawful Development Certificate (£274) to confirm PD eligibility.
How long does a house extension take in Barnet?
A typical house extension project in Barnet takes 5-8 months from initial instruction to completion. This breaks down as: 2-3 weeks for measured survey and drawing preparation, 8 weeks for planning determination by Barnet Council (if required), 2-4 weeks for building regulations approval, and 8-16 weeks for construction depending on the size and complexity of the extension.
Do I need a Party Wall Agreement for an extension in Barnet?
Barnet is mostly semi-detached, so a rear extension normally involves a single adjoining owner rather than two. Foundations within three metres of theirs, or any work to the shared wall itself, are notifiable under the Party Wall etc. Act 1996. Where the neighbour dissents, surveyors are appointed and their fees fall to you, so it belongs in the budget rather than the risk register.
What experience do you have with extensions in Barnet?
We prepare extension drawings for properties across all Barnet postcodes (N2, N3, N10, N11, N12, N14, N20, NW2, NW4, NW7, NW9, NW11, EN4, EN5). Our MCIAT chartered architectural technologists understand Barnet Council's policies, design guides, and local precedents. We have a council-ready drawing review across all 33 London boroughs.
