Side extensions 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 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 Hillingdon
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 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
Hillingdon officers apply the HDAS Residential Extensions guidance closely and are firm on set-backs, depth and the 45-degree test. 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.
