Two-storey extensions for detached houses in Friern Barnet

Quick reference for homeowners in Friern Barnet.

Postcode districts: N11 and N12
County / area: Greater London, England
Price region: London
Guide price: £103,500–£138,500 (£2,460–£3,300 per m²)
Approval route: Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); a two-storey side extension needs planning permission.
Nearest towns: Muswell Hill (1.7 miles), Finchley (1.8 miles), Wood Green (2 miles)
First visit: Free, no obligation

About a two-storey extension for a detached house

This service suits you if you own a detached house and want a substantial amount of extra space, whether a side extension over a drive, a rear extension into the garden or a wraparound. You get a survey of the whole plot, options that compare layouts and costs, the approvals your nation needs, structural design and the build, carried out by vetted teams working under S&K.

We cover postcode districts N11 and N12 in and around Friern Barnet, along with neighbouring Muswell Hill, Finchley and Wood Green.

What it costs in Friern Barnet

A guide price for Friern Barnet: around £103,500–£138,500 for a 3 m × 7 m footprint on two floors (42 m² of new floor space). That works out at £2,460–£3,300 per m² of new floor space, including VAT at 20%. Keep a contingency of around 10% for things nobody can see until the ground is opened up.

Spec	Per m²	A 3 m × 7 m footprint on two floors (42 m² of new floor space)
Essential	£2,260–£3,040 per m²	£95,000–£127,500
Standard	£2,460–£3,300 per m²	£103,500–£138,500
Premium	£2,950–£3,960 per m²	£124,000–£166,500

Planning and building regulations in Friern Barnet

In England, a two-storey rear extension on a detached house can be permitted development within 3 m of the original rear wall and at least 7 m from the rear boundary, with matching roof pitch and materials, outside protected areas. Any two-storey side or wraparound extension needs planning permission, costing £548 (or £575 for applications made from 8 December 2026), with a decision due within 8 weeks of validation.

Approval in England: Householder planning application: £548 (or £575 for applications made from 8 December 2026). Lawful development certificate: £274 (about £288 for applications from 8 December 2026). Councils should decide householder applications within 8 weeks of validation.

Building regulations: Building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end.

Design points for Friern Barnet homes

Choosing the best side

On a detached house you can often extend on either side or the rear. We look at orientation for daylight, where the stairs and landing are, the distance to each boundary, drains and trees, so the extension goes where it adds the most useful space.

Keeping it subordinate

Large detached houses can take a large extension, but councils still expect it to look secondary to the main house. A lower ridge, a modest set-back at the front and matching materials keep the proportions right, even on a wide plot.

How we'd run your project

Measure and listen
A visit to Friern Barnet to understand how you live in the house now and what the extra rooms need to do, plus a first look at the structure.
Design and approvals
We draw the extension and check it against the rules for England, then apply for planning permission or a certificate of lawfulness, whichever fits.
Technical design
We prepare the building regulations drawings, structural calculations and specification so the build price can be fixed.
On site
One project lead runs the job, keeps you updated and arranges inspections at each stage until completion.

Questions about two-storey extensions for detached houses in Friern Barnet

How long does a two-storey extension for a detached house take?

Allow 4 to 8 weeks for survey and design, then the approval period (councils should decide householder applications within 8 weeks of validation), then 4 to 8 weeks of technical design, which can overlap. The build itself usually takes 14 to 20 weeks, longer for large or complex designs.

Who handles building regulations for a project in Friern Barnet?

We handle it as part of the project. What's needed in England: building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end. We prepare the drawings and the engineer's calculations, book the inspections and pass you the completion paperwork at the end.

Which postcodes do you cover around Friern Barnet?

We cover Friern Barnet (N11 and N12) and nearby places including Muswell Hill, Finchley and Wood Green, plus the rest of Greater London. The first site visit is free.

Is a detached house extension a better investment than moving?

It often costs less than the gap between your house and a larger one nearby, once property tax, legal fees and removal costs are counted, but that depends on your area. Extending also has limits: every street has a ceiling price. We give you build costs so you can compare them with local asking prices.

How big an extension can a detached house take?

It depends on the plot, the boundaries and local design policy rather than a fixed limit. Under permitted development in England, extensions must not cover more than 50% of the land around the original house. With planning permission, the council judges size on proportion, neighbours and the character of the area.

Nearby

The same service in neighbouring towns:

Other extensions in Friern Barnet

All two-storey extension services in Friern Barnet · Two-storey extensions for detached houses across Greater London

The quickest next step is a visit. We'll look at your house in Friern Barnet and give you an honest view of the options.