TL;DR — Key Facts
- Build cost per sqm: £2,200 – £3,200 (outer London borough)
- Our drawing fees: Essentials from £1,225 · Complete from £1,750
- Council planning fee: £258 (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
Extension costs 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.
The actual cost of your extension will depend on the size, specification, and complexity of the project. A basic rear extension with standard finishes will sit at the lower end, while a high-specification kitchen extension with bi-fold doors, underfloor heating, and bespoke joinery will sit at the upper end or above.
Typical extension costs in Barnet (build only)
On top of the build cost, you will need architectural drawings. Our Essentials package starts from £1,225 for extension drawings including existing and proposed plans, elevations, and a site plan. Our Complete package from £1,750 includes full planning and building regulations drawings. These fees are 30% below typical London architect rates.
What affects extension costs in Barnet
Several local factors affect extension costs in Barnet. Understanding these before you start will help you budget accurately and avoid surprises.
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 (£258 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
Terraced properties in Barnet often have limited rear access, which can increase build costs by £3,000-8,000 due to the need to transport materials through the house. Properties with side access or rear lane access avoid this premium. Parking restrictions and CPZ zones in Barnet can also add skip permit and parking suspension costs of £200-500.
Our drawing fees for Barnet extensions
At Architectural Drawings London, we offer fixed-fee packages for house extension drawings. 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 vs 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).
Properties in Barnet's 20 conservation areas have more restricted PD rights. In conservation areas, you cannot build side extensions under PD, rear extensions are more limited, and cladding changes require planning permission.
Two-storey extensions almost always require full planning permission. Barnet Council generally expects two-storey extensions to be subordinate to the original dwelling, set in from the side boundary, and designed to avoid unacceptable loss of light or privacy to neighbouring properties.
We recommend obtaining a Lawful Development Certificate (£129) even for PD projects. This provides legal proof that your extension is lawful and is valuable when selling the property. We prepare the LDC drawings and application on your behalf.
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, you need architectural drawings (from £1,225 with us) and a council planning fee of £258 if planning permission is required.
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 (£129) 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?
If your extension in Barnet involves building on or near a shared boundary wall, excavating within 3 metres of a neighbour's foundation, or excavating within 6 metres to a depth below their foundation, you will need a Party Wall Agreement under the Party Wall etc. Act 1996. This is common for side-return extensions on terraced properties in Barnet. A Party Wall surveyor typically costs £700-1,500 per neighbour. We can recommend Party Wall surveyors who work in Barnet.
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 98% first-time approval rate across all 33 London boroughs. Our fixed fees start from £1,225 for extensions, which is 30% below typical London architect rates.