The permitted development rules for garden rooms and outbuildings
An outbuilding is permitted development on a house if it is single storey, no higher than 2.5 m where it comes within 2 m of a boundary, otherwise no higher than 4 m with a dual-pitched roof or 3 m with any other roof, sits behind the front wall of the house, covers together with other outbuildings no more than half the land around the original house, and is used for purposes incidental to the house rather than as a bedroom or self-contained flat.
Anything used for sleeping or as a separate dwelling needs planning permission regardless of size. Flats have no permitted development rights for outbuildings, and listed buildings need listed building consent.
What changes in Hounslow
In a conservation area an outbuilding is still permitted development behind the house, but not to the side of it, and the same height limits apply.
The local constraints that most often decide the route in Hounslow are listed below, taken from our Hounslow area page. Each one is a reason to confirm the address on Hounslow's constraints map before relying on permitted development.
- Conservation areas. Hounslow has numerous conservation areas, concentrated in Chiswick (including Bedford Park, Strand-on-the-Green and Chiswick House), Brentford and Old Isleworth. Roof extensions and side extensions in these areas need planning permission rather than permitted development. Check the council's constraints map before assuming permitted development rights apply. Source
- Article 4 directions. Bedford Park has long-standing Article 4 directions removing permitted development rights for external alterations, so even minor changes to windows, roofs and front gardens need permission. Other conservation areas in the borough may also be covered; ask the council or check the constraints map before starting design.
- Thames flood zones. Land close to the Thames at Chiswick, Brentford and Isleworth falls within Flood Zones 2 and 3. Basements and habitable rooms at ground level may need a flood risk assessment even for a householder application. Source
- Green Belt and Metropolitan Open Land. Osterley Park, parts of Hanworth, Cranford and the Duke of Northumberland's River corridor are Green Belt or Metropolitan Open Land. Outbuildings and extensions to houses inside or adjoining these designations are judged more strictly on openness.
What Hounslow planning officers look for
Hounslow generally follows the London-wide conventions for householder extensions: single-storey rear extensions of 3 to 4 m are usually acceptable where they respect the 45-degree rule to neighbours' windows, and refusals most often cite excessive depth, loss of daylight or bulk in two-storey schemes. In Chiswick's conservation areas officers expect traditional materials and resist front dormers and boxy roof extensions. Side extensions on 1930s semis are expected to be set back and subordinate so the gap between houses is not closed. Pre-application advice is available for a fee and is worth using for anything in Bedford Park or on the riverside.
Fees and timelines
The council fee for a householder application is £528 and for a Lawful Development Certificate £264 (England, from 1 April 2025; verify before applying). Both are decided within 8 weeks of validation.
Our own fee for the drawings and the application is fixed once quoted and depends on the project type, its size and the constraints above. Answer six questions in the quote tool and we email you an indicative figure within minutes, then a written fixed-fee quote within 24 hours.
