Home Extension Design in Kensington and Chelsea: Planning, Costs and What's Realistic in 2026
A home extension in Kensington and Chelsea typically costs £4,000 to £5,500 or more per m² to build in 2026, at mid-range specification, before VAT and professional fees. Planning is rarely a formality here. The Royal Borough has 38 conservation areas covering nearly three quarters of its streets, dozens of Article 4 Directions controlling details as small as a front door, and one of the strictest basement policies in London. This page covers what the council allows, what it costs, and how to find out where your street stands before design work begins.
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 Kensington and Chelsea?
Almost certainly yes. Kensington and Chelsea has 38 conservation areas protecting nearly three quarters of the borough, so most homes sit inside one. Within a conservation area, side extensions, rear extensions beyond 3 metres and any rear extension of more than one storey need planning permission. On top of that, the council maintains street-by-street Article 4 Directions, made between 1969 and 2014, which remove permitted development rights for smaller works such as changing windows, front doors, roof coverings and boundary walls, often on front elevations.
The housing stock explains the caution. Stucco-fronted terraces and garden squares in Chelsea, Italianate villas in Holland Park, the planned Victorian layouts of the Norland, Ladbroke and Pembridge conservation areas around Notting Hill, and tall red-brick terraces in Earl's Court all carry protections that vary street by street. Chelsea Square gained its own Article 4 Direction in 2003, Addison Avenue in 2012, and there are many more.
Outside the conservation areas, the ordinary national rules apply: 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. In this borough, though, never assume that until someone has checked your address against the council's conservation area map and Article 4 schedule. That check is part of our Stage Zero review, before any design work starts.
What are the rules for basement extensions in Kensington and Chelsea?
Basements in Kensington and Chelsea are limited to a single storey and to no more than half of the garden under most homes. That is Policy CD11 of the council's Local Plan, adopted in July 2024, which carries forward and tightens the approach first set by Policy CL7 in 2015. The single storey must be genuinely single: the policy is written so it cannot later be subdivided horizontally.
Permitted development is no help either. The council made a borough-wide Article 4 Direction in April 2015, in force from 28 April 2016, removing permitted development rights for basement extensions to houses. The Secretary of State modified it in July 2022 to cover a smaller area, but across much of the borough a basement still needs a full planning application, typically with structural and construction management information alongside the drawings.
None of this makes basements impossible. It makes them slower and more document-heavy, and it rewards a feasibility check before you commit to the idea. A single-storey basement designed within CD11 from the outset has a far smoother path than a scheme trimmed down after a refusal.
How much does a home extension cost in Kensington and Chelsea in 2026?
Kensington and Chelsea sits in London's most expensive 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 Kensington and Chelsea costs £4,000 to £5,500 or more per m² at mid specification in 2026. A loft conversion runs £3,200 to £3,800 per m². Basements are the big number: £6,000 to £8,000 or more per m² under a garden, and £8,000 to £12,000 or more per m² under the house itself. Conservation area projects carry roughly 15% on top for material restrictions and period-appropriate detailing, which in this borough applies to most streets. 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 by extension type and specification level is in our free Extension Cost Guide 2026, no email required.
To see how we design and deliver work like this, have a look at the Rathcoole Gardens project page and the Drakefield House project page, both completed London projects with the process shown from brief to build.
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 Kensington and Chelsea 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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