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 Bromley
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 Bromley are listed below, taken from our Bromley area page. Each one is a reason to confirm the address on Bromley's constraints map before relying on permitted development.
- Conservation areas. Bromley has numerous conservation areas, including Chislehurst, Petts Wood, Bickley Park, Beckenham Place, Sundridge Park, Keston and several around Bromley town centre. Roof extensions are not permitted development inside them and extensions are expected to be clearly subordinate. Check the council's constraints map for your address. Source
- Green Belt and Metropolitan Open Land. Roughly half of the borough is Green Belt or Metropolitan Open Land, taking in Keston, Downe, Cudham, Biggin Hill, Chelsfield and Pratts Bottom. Extensions must not be disproportionate to the original house, and outbuildings are judged strictly. We check the designation before drawing. Source
- Article 4 directions. Several Bromley conservation areas, including parts of Chislehurst and Petts Wood, have Article 4 directions removing permitted development for front-facing alterations, boundary treatments or roof works. The exact restrictions vary by street, so confirm on the council's constraints map before building under PD. Source
- Flood risk – Ravensbourne, Cray and Beck. The River Ravensbourne, Cray, Beck and Pool River corridors through Beckenham, Shortlands, Bromley and St Mary Cray carry fluvial flood risk. Ground-floor extensions in Flood Zones 2 and 3 may need a flood risk statement. Source
What Bromley planning officers look for
Bromley officers are protective of suburban spaciousness. Refusals most often cite two-storey side extensions that close the gap between houses or lack a set-back, rear extensions that breach the 45-degree line to a neighbour, and roof extensions in conservation areas or Areas of Special Residential Character. Rear dormers on ordinary semis outside protected areas are usually accepted if they sit below the ridge and inside the roof slope. The council generally offers a paid pre-application service; 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.
