Garden rooms and outbuildings in Hillingdon depend on the house. Typical homes here: 1930s semi-detached houses in Ruislip, Eastcote, Ruislip Manor, Northwood Hills, Ickenham, Hillingdon and Hayes; Detached inter-war houses in Northwood, Ruislip, Ickenham and around Ruislip Woods. Designations to check first: Green Belt; Conservation areas; Article 4 directions. Applications are decided by London Borough of Hillingdon and the Hillingdon Local Plan (Part 1 2012 and Part 2 2020).
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 Hillingdon
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 Hillingdon are listed below, taken from our Hillingdon area page. Each one is a reason to confirm the address on Hillingdon's constraints map before relying on permitted development.
- Green Belt. Large areas of Hillingdon are Green Belt, including Harefield, the Colne Valley, Ruislip Woods and the fringes of Ickenham and Northwood, and the Heathrow villages of Harmondsworth, Sipson and Longford. Source
- Conservation areas. Hillingdon has around 30 conservation areas, including Harefield Village, Ruislip Village, Eastcote Village, Ickenham Village, Old Uxbridge and Windsor Street, Harmondsworth Village, Cowley Church and Northwood Frithwood. Source
- Article 4 directions. Several conservation areas have Article 4 directions removing permitted development for front alterations, windows and boundary treatments, and the council has pursued a borough-wide direction on small HMOs. Source
- River and canal flood risk. The River Colne, Frays River, River Pinn, Yeading Brook and the Grand Union Canal put parts of West Drayton, Yiewsley, Cowley, Harefield, Uxbridge, Ruislip and Hayes in Flood Zones 2 and 3. Source
What Hillingdon planning officers look for
Common refusal reasons are two-storey side extensions without a 1 m set-back that create a terracing effect, rear extensions deeper than the guidance allows against a neighbour, front dormers, box dormers that wrap the hip, and outbuildings in the Green Belt. The council offers paid pre-application advice and decisions often run beyond 8 weeks, so allow 10–14 weeks.
Fees and timelines
Hillingdon 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 Hillingdon: planning: the council's own constraints map, validation checklist and pre-application service.
