Home Extension Design in Barnet: Planning, Costs and What's Realistic in 2026

A home extension in Barnet typically costs £3,200 to £4,500 per m² to build in 2026, at mid-range specification, before VAT and professional fees. Most of the borough's interwar semis sit in ordinary permitted-development territory, so plenty of extensions need no planning application at all. The exceptions matter, though: Barnet has 16 conservation areas, and in Hampstead Garden Suburb a 1971 Article 4 Direction plus a separate Trust approval process makes almost every external change a consented one. This page covers what the council allows and what it costs.

Studio MASS is a residential design practice led by an architectural technologist. We design and deliver extensions, loft conversions and refurbishments across London, from first feasibility check to completed build.

Do you need planning permission for an extension in Barnet?

Many Barnet extensions do not need planning permission. Outside conservation areas, a single-storey rear extension up to 3 metres beyond the original rear wall (4 metres on a detached house) usually falls under permitted development, and that covers a large share of the borough's 1930s semis in Finchley, Hendon and Whetstone. Inside a conservation area, side extensions and rear extensions beyond 3 metres, and any rear extension of more than one storey, need planning permission.

Barnet has 16 conservation areas, including Hampstead Garden Suburb, Totteridge, Monken Hadley, Mill Hill, Golders Green and Finchley Church End. Article 4 Directions in Hampstead Garden Suburb and Totteridge go further, removing permitted development rights for house enlargements, roof alterations, porches, outbuildings, hardstandings and boundary walls. Householder schemes borough-wide are assessed against Barnet's Residential Design Guidance SPD, adopted in October 2016.

What if your house is in Hampstead Garden Suburb?

Hampstead Garden Suburb runs on a double consent system. An Article 4 Direction, first made in 1971 and since updated, means planning permission is needed for works that would be permitted development anywhere else: extensions, roof alterations, porches, sheds, hardstandings, gates and fences, even repainting external walls. On top of that, the Hampstead Garden Suburb Trust operates a Scheme of Management approved by the High Court in 1974, so most external alterations also need separate Trust approval.

None of this makes extending in the Suburb impossible. It makes the design and application work heavier, and it rewards proposals drawn up against the joint Council and Trust design guidance from the start. Budget more time, and expect period-appropriate materials to be non-negotiable.

How much does a home extension cost in Barnet in 2026?

Barnet sits in London's mid-price construction zone. From our 2026 Extension Cost Guide, built from BCIS indices, FMB build cost data and our own completed projects:

A single-storey extension in Barnet costs £3,200 to £4,500 per m² at mid specification in 2026. A loft conversion runs £3,200 to £3,800 per m². Conservation area projects carry roughly 15% on top for material restrictions and period-appropriate detailing, and Hampstead Garden Suburb work tends toward the upper end. These figures are construction only; VAT at 20% and professional fees are additional.

Before a brick is laid, expect £8,000 to £16,000 in professional and statutory fees on a typical project, including the £548 planning application fee that applies from April 2026.

The full breakdown is in our free Extension Cost Guide 2026, no email required. For a sense of how we work and what finished projects cost, see the Rathcoole Gardens project page and the Drakefield House project page, both completed London projects with real budgets.

Who designs your extension if not an architect?

An architectural technologist designs buildings with a focus on how they are built: construction detail, building regulations, and buildability, alongside layout and light. In the UK "architect" is a legally protected title, so we don't use it. What matters for a Barnet extension is the same either way: design within the local planning context, technical drawings a builder can price accurately, and someone accountable from first idea to finished build. That is what we do, through one team and five stages, starting with a Stage Zero review that tells you what is realistic for your budget and what planning will allow before design begins.

Start with a conversation

The best starting point is our short questionnaire. It takes a few minutes and tells us about your home, your brief and your budget. We review it and arrange a free thirty-minute call, with an honest view on feasibility either way.

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