Planning Permission in Hillingdon for Extensions & Lofts (2026 Guide)

If you own a house in Hayes, Uxbridge, Ruislip, Ickenham, Northwood, West Drayton or anywhere else in the London Borough of Hillingdon and you’re thinking about an extension or a loft conversion, the first question is always the same: do I need planning permission from Hillingdon Council? Often the answer is no – but the rules that decide it are specific, the fees changed again in April 2026, and Hillingdon has a few local wrinkles (conservation areas, Article 4 directions, Heathrow) that catch people out. This guide, written by a Hayes-based builder, walks through it in plain English.

The three routes: permitted development, prior approval, full planning

Every householder project in Hillingdon goes down one of three routes. Which one applies depends on what you’re building, how big it is, and whether your house has any restrictions attached to it.

The fees above are the national figures set by government and charged by Hillingdon Council; they were indexed on 1 April 2026 (householder £528 → £548, prior approval £240 → £249). Your architect or builder will add their own fee for preparing the drawings and submitting.

What you can build in Hillingdon without planning permission

Permitted development rights are the same across England, so a 1930s semi in Hayes has the same allowances as one in Harrow. The limits that matter most for extensions and lofts:

• Single-storey rear extension: up to 3 m deep on a semi or terrace, 4 m on a detached house; max 4 m high (3 m if within 2 m of a boundary); no more than half the garden covered.
• Larger rear extension (prior approval): up to 6 m on a semi/terrace or 8 m detached, single storey only, after a 21-day neighbour consultation.
• Two-storey rear extension: up to 3 m deep, must be at least 7 m from the rear boundary – rarely achievable on Hillingdon’s shorter gardens, so most go to full planning.
• Side extension: single storey, up to half the width of the original house, max 4 m high.
• Loft conversion: up to 40 m³ extra roof volume on a terrace, 50 m³ on a semi or detached; rear dormers allowed, no dormers on the front roof slope facing the road; materials to match.
• Outbuildings: garden rooms and garages up to 2.5 m eaves height within 2 m of a boundary.

Two things remove those rights: living in a conservation area, or an Article 4 direction. Hillingdon has around 30 conservation areas, including Harefield Village, Ickenham Village, Ruislip Village, Eastcote Village, Harmondsworth Village, Old Uxbridge and parts of Northwood – in these, side extensions, roof extensions and cladding all need planning permission, and the council will look at materials closely. Hillingdon has also used Article 4 directions in some areas; the council publishes the current list on its planning pages. If you’re unsure, the council’s free duty planner will tell you by phone.

The Heathrow factor

Much of southern Hillingdon – Hayes, Harlington, Harmondsworth, Sipson, West Drayton – sits under Heathrow’s flight paths. That doesn’t change planning rules, but it does change Building Regulations in practice: Part E (sound) and Part L (energy) mean acoustic glazing and better roof insulation are usually worth specifying on a loft or extension here, and Building Control will expect ventilation that works with windows closed. It also affects value – a well-insulated, quiet loft bedroom is a genuine selling point in Hayes.

How to apply to Hillingdon Council

All applications go through the Planning Portal (planningportal.co.uk), which forwards them to Hillingdon. What you need:

• Location plan (1:1250) and block plan (1:500) – bought from the Planning Portal or drawn by your architect
• Existing and proposed floor plans and elevations, to scale
• The application form and fee (£548 householder, £249 prior approval)
• For prior approval: the depth, height and eaves height of the extension, and addresses of adjoining neighbours
• In a conservation area: a short design and access statement and materials schedule

Hillingdon offers a free duty planner (telephone appointment) for householder questions, and paid written pre-application advice – £100 for a follow-up letter on a householder scheme – with a response within 15 working days. For a straightforward extension we usually skip pre-app and apply; for anything in a conservation area it’s worth the £100.

How long it really takes in Hillingdon

Work backwards from when you want to start on site. A rear extension that needs full planning in Hillingdon is realistically 4–5 months from first sketch to first dig; a permitted-development extension can be 6–8 weeks. Party Wall notices (see our party wall guide) run alongside and are often the real bottleneck on semis.

Building Regulations – the bit you can’t skip

Whether or not you need planning permission, every extension and loft conversion needs Building Regulations approval and a completion certificate. You can use Hillingdon Building Control or an approved inspector; either way the inspections cover foundations, damp-proofing, structure, insulation, fire safety, drainage and electrics. Without the certificate you will struggle to sell the house – solicitors ask for it – so make sure your builder is including it, as GSM Builders Ltd does on every job.

Common reasons Hillingdon refuses householder applications

• Two-storey extensions that overshadow or overlook the neighbour’s garden (the 45-degree rule)
• Side extensions that close the gap between semis and create a terraced look
• Front extensions or dormers on the road-facing roof slope
• Loss of off-street parking without replacement – a big one in Hayes and Uxbridge
• Flat-roof dormers that are too big for the roof in conservation areas

Almost all of these are avoidable at the drawing stage. The cheapest planning consultant you will ever hire is a builder who has put twenty extensions through the same council and knows what the case officers push back on.

Frequently asked questions