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. Extensions to houses in the Green Belt must not be disproportionate to the original dwelling, and outbuildings face a strict test. 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. Roof extensions are not permitted development inside them and extensions are expected to be subordinate and in matching materials. 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. Check the constraints map and the current position before relying on permitted development for a conversion. 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. Ground-floor extensions there may need a flood risk statement. 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. Hip-to-gable lofts and 4–6 m rear extensions on ordinary 1930s semis are usually accepted. The council offers paid pre-application advice and decisions often run beyond 8 weeks, so allow 10–14 weeks.
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.
