“Do I need planning permission?” is the question we are asked at almost every first site visit in West London. The answer is often no – but it depends on what you are building, what kind of house you have and whether your street has extra restrictions. This guide explains the rules as they apply in Hillingdon, Ealing, Hounslow and Harrow in 2026, in plain English.
The three routes to building legally
- Permitted development (PD) – rights granted nationally that let you extend or convert without applying, provided you stay within set limits.
- Prior approval – a lighter-touch process for larger single-storey rear extensions (the “larger home extension scheme”), where the council consults neighbours but cannot refuse on design grounds.
- Full householder planning application – needed when you exceed PD limits, live in a flat, or your permitted development rights have been removed.
Whichever route applies, Building Regulations approval is always required – planning and Building Control are two separate things.
Single-storey rear extensions
| House type | Permitted development | With prior approval |
|---|---|---|
| Terraced or semi-detached | Up to 3 m deep | Up to 6 m deep |
| Detached | Up to 4 m deep | Up to 8 m deep |
Other PD conditions: maximum height 4 m (3 m if within 2 m of a boundary) for a flat roof, eaves no higher than the existing eaves, materials similar in appearance to the house, and the extension must not cover more than half the garden. Most 1930s semis in Hayes, Hounslow, Ruislip and Greenford can take a 3 m or even 6 m rear extension without a planning application.
Side extensions
A single-storey side extension is permitted development if it is no wider than half the width of the original house and no higher than 4 m. Side-return extensions on Victorian terraces in Ealing and Acton usually qualify – unless the house is in a conservation area.
Double-storey and wrap-around extensions
Two-storey rear extensions can be permitted development if they extend no more than 3 m and are at least 7 m from the rear boundary, with matching roof pitch and obscure-glazed side windows upstairs. In practice, on most West London plots the 7 m rule fails, so double-storey extensions almost always need a planning application. Wrap-arounds also normally need one because side and rear extensions cannot be combined under PD.
Loft conversions
Permitted development allows up to 40 m³ of additional roof space on a terrace and 50 m³ on a semi or detached house (including any previous roof extensions). The dormer must be at the rear or side, not the front, must sit at least 20 cm above the eaves, and must not rise above the ridge. Hip-to-gable and rear dormer conversions on 1930s semis usually fit within 50 m³.
When you definitely need planning permission
- Flats and maisonettes – no permitted development rights at all.
- Conservation areas with Article 4 directions – PD rights for extensions, dormers and cladding are often removed.
- Listed buildings – listed building consent needed for almost any alteration.
- Houses already extended – PD limits apply to the original house as built, so previous extensions count.
- Front extensions or front dormers.
- Raising the ridge height.
- New houses built under a planning condition that removed PD rights – common on estates built since the 1990s.
Local rules: Hillingdon, Ealing, Hounslow and Harrow
London Borough of Hillingdon
Covers Hayes, Hillingdon, Uxbridge, Ruislip, West Drayton, Harmondsworth and Sipson. Conservation areas include Old Uxbridge, Ruislip Village, Eastcote Village, Harmondsworth Village and Harefield. Hillingdon has a well-organised Lawful Development Certificate process and a standard fee of £129 for a householder LDC (half the application fee). Their householder application fee is £258. See our dedicated guide to house extensions in Hillingdon.
London Borough of Ealing
Covers Ealing, Acton, Hanwell, Southall, Greenford, Northolt and Perivale. Ealing has over 25 conservation areas, many with Article 4 directions removing PD for rear extensions and roof alterations – Ealing Common, Ealing Green, Bedford Park (shared with Hounslow), Acton Green and Hanger Hill Garden Estate among them. Always check the council’s conservation area map before assuming PD applies.
London Borough of Hounslow
Covers Hounslow, Heston, Cranford, Isleworth, Feltham and Chiswick. Conservation areas include Spring Grove, Isleworth Riverside, Hounslow Heath and Chiswick. Hounslow encourages prior approval for larger rear extensions and publishes a useful householder design guide.
London Borough of Harrow
Covers Harrow, Kenton, Rayners Lane, Pinner, Wealdstone and Stanmore. Harrow on the Hill, Pinner High Street and Harrow Weald Park are the main conservation areas, with Article 4 directions in Harrow on the Hill.
How long does planning take?
| Route | Statutory timescale | Typical 2026 fee |
|---|---|---|
| Prior approval (larger home extension) | 42 days | £120 |
| Householder planning application | 8 weeks | £258 |
| Lawful Development Certificate | 8 weeks | £129 |
| Building Regulations (full plans) | 5–8 weeks | £800 – £1,500 |
What happens if you build without permission?
The council can serve an enforcement notice requiring the work to be altered or removed, and unauthorised work will surface in a buyer’s searches when you sell. It is far cheaper to get the paperwork right first. GSM Builders Ltd handles planning, prior approval and Lawful Development Certificates as part of every extension and loft project.
Not sure which route applies to your house?
Send us your address and a description of what you want to build. We will check the conservation area maps, planning history and PD limits for your property and tell you – free – whether you need to apply. Read more about our house extensions and loft conversions, or see our guides to extension costs and loft conversion costs in West London.
