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 Lewisham
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 Lewisham are listed below, taken from our Lewisham area page. Each one is a reason to confirm the address on Lewisham's constraints map before relying on permitted development.
- Conservation areas. Lewisham has numerous conservation areas, including large ones at Brockley and Blackheath and others in Forest Hill, Sydenham, Lee Manor, Telegraph Hill and Deptford. Roof extensions and cladding in these areas need planning permission rather than permitted development. Check the council's constraints map for your address before assuming PD applies. Source
- Article 4 directions. The Brockley conservation area and some others in Lewisham carry Article 4 directions removing permitted development for front alterations, windows, roofs and boundary treatments. Coverage varies by area, so we check the schedule for your street before recommending a permitted development route.
- Ravensbourne, Quaggy and Pool river flood zones. The Ravensbourne, Quaggy and Pool rivers run through Catford, Lewisham town centre, Ladywell, Lee and Bellingham, and land along them falls within Flood Zones 2 and 3. The Thames frontage at Deptford is also Flood Zone 3. Basements and new ground-floor habitable rooms there may need a flood risk assessment. Source
- Listed buildings in Blackheath and Deptford. Blackheath, Blackheath Park, Deptford High Street and St Paul's Deptford contain many listed Georgian and Victorian buildings. Listed houses need listed building consent for almost any alteration, and extensions to neighbours are judged on their effect on the setting.
- Metropolitan Open Land. Blackheath, Hilly Fields, Beckenham Place Park, Mountsfield Park and the Waterlink Way corridor are Metropolitan Open Land. Houses fronting or backing onto them are judged on the effect of extensions and outbuildings on openness and setting.
What Lewisham planning officers look for
Lewisham's officers apply the Alterations and Extensions SPD fairly closely. Rear extensions deeper than neighbouring precedent, side returns that breach the 45-degree rule and roof extensions in Brockley, Blackheath and other conservation areas are the common refusal reasons; front dormers are resisted almost everywhere. On 1930s streets in Grove Park and Downham side extensions are expected to be set back and subordinate. Lewisham is a busy authority and householder decisions can run past the statutory period. Pre-application advice is available for a fee.
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.
