Side extensions in Hounslow depend on the house. Typical homes here: Edwardian and Arts and Crafts houses in Bedford Park and Chiswick; Victorian terraces in Brentford, Old Isleworth and around Chiswick High Road. Designations to check first: Conservation areas; Article 4 directions; Thames flood zones. Applications are decided by London Borough of Hounslow and the Hounslow Local Plan 2015–2030.
The permitted development rules for side extensions
A single-storey side extension is permitted development on a house if it is no wider than half the width of the original house, no higher than 4 m, with eaves no higher than 3 m where it comes within 2 m of a boundary, and finished in materials similar in appearance to the house.
Anything wider than half the original house, any two-storey side extension, and any side extension on a flat needs planning permission. Garage conversions that keep the walls are usually permitted development; extending beyond them is assessed as a side extension.
What changes in Hounslow
Side extensions are not permitted development in a conservation area at all, so there a planning application is always required.
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. 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.
- Thames flood zones. Land close to the Thames at Chiswick, Brentford and Isleworth falls within Flood Zones 2 and 3. 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.
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
Hounslow decides a householder application or a Lawful Development Certificate within 8 weeks of validation. The council fee is £528 for a householder application and £264 for a Lawful Development Certificate (England, from 1 April 2025; verify before applying).
Our own fee is fixed once quoted. The quote tool gives an indicative figure in minutes and a written quote follows within 24 hours.
Primary sources
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2: the permitted development limits quoted above.
- Planning Portal: do you need permission: the national guidance and the application service every council accepts.
- London Borough of Hounslow: planning: the council's own constraints map, validation checklist and pre-application service.
