Home Extension Design in Hillingdon: Planning, Costs and What's Realistic in 2026
A home extension in Hillingdon typically costs £2,800 to £3,800 per m² to build in 2026, at mid-range specification, before VAT and professional fees. Planning is where this borough differs from most of London: a borough-wide Article 4 Direction means any single-storey rear extension deeper than 4 metres needs a full planning application, wherever you live, and 31 conservation areas tighten the rules further street by street. This page covers what the council allows, what it costs, and how to find out where your house stands.
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 Hillingdon?
Many Hillingdon extensions do not need planning permission, but the borough has one rule most of London does not. Since 22 September 2014, an Article 4 Direction covering the whole borough has removed permitted development rights for single-storey rear extensions that go more than 4 metres beyond the original rear wall. The larger home extension route, which lets homeowners elsewhere build to 6 or 8 metres with prior approval, does not operate in Hillingdon. Up to 3 metres (4 metres on a detached house), the usual permitted development allowances still apply outside conservation areas.
Inside a conservation area the position tightens again. Hillingdon has 31 designated conservation areas, from old village centres such as Ruislip Village, Eastcote Village, Ickenham Village and Harefield Village to Old Uxbridge/Windsor Street and a string of canal-side areas along the Grand Union. In a conservation area, side extensions, rear extensions beyond 3 metres and any rear extension of more than one storey need planning permission.
When an application is needed, the council assesses it against its adopted residential extensions design guide, HDAS: Residential Extensions (adopted December 2008), which sets expectations on matters such as garden depth retained behind the extension and keeping outbuildings subordinate to the main house.
The practical rule: never assume permitted development applies in Hillingdon until someone has checked your street against the council's heritage asset map and the Article 4 position. That check is part of our Stage Zero review, before any design work starts.
What kinds of homes get extended in Hillingdon?
Most of our Hillingdon enquiries concern interwar houses. Ruislip, Eastcote and Ickenham grew up around the railway between the wars, and their streets of 1920s and 1930s semis and detached houses, with generous plots and hipped roofs, suit rear extensions and loft conversions well. The council itself describes its conservation areas as ranging from old village centres to planned residential estates, and two of those estates, Eastcote Park and Morford Way in Eastcote, are protected for exactly that interwar character.
Around the old village cores the picture is older and more sensitive. Ruislip Village, first designated in 1969, Ickenham Village and Harefield Village all carry conservation area status, so extensions there need period-appropriate materials and a planning application sooner than elsewhere. Uxbridge mixes its historic core around Old Uxbridge/Windsor Street with ordinary suburban streets, while Hayes and West Drayton offer some of the borough's best value, with terraced and semi-detached stock and their own pockets of protection at Hayes Village and West Drayton Green.
How much does a home extension cost in Hillingdon in 2026?
Hillingdon sits in London's outer, lower-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 Hillingdon costs £2,800 to £3,800 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. 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.
If you want to see how we work, two completed London projects show the range: a side-return kitchen extension at Rathcoole Gardens project page in Hornsey, north London, and a whole-house project at Drakefield House project page. The process is the same wherever the postcode.
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 Hillingdon 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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