The permitted development rules for loft conversions
A loft conversion with a rear dormer is permitted development on a house if the added roof volume is no more than 40 m³ on a terraced house or 50 m³ on a semi-detached or detached house, the dormer sits at least 200 mm back from the eaves and no higher than the ridge, there is no dormer on the roof slope facing the road, materials are similar in appearance, and side windows are obscure-glazed.
Flats and maisonettes have no permitted development rights, so a loft to a top-floor flat always needs planning permission. Full-width box dormers that reach the ridge, and hip-to-gable conversions on semis whose volume exceeds 50 m³, also fall outside the allowance.
What changes in Merton
Roof extensions are not permitted development in a conservation area, so a dormer or hip-to-gable there needs a householder planning application designed to the area's appraisal.
The local constraints that most often decide the route in Merton are listed below, taken from our Merton area page. Each one is a reason to confirm the address on Merton's constraints map before relying on permitted development.
- Conservation areas. Merton has numerous conservation areas, concentrated in Wimbledon Village, Wimbledon Park, Merton Park, Mitcham Cricket Green and along the Wandle. Roof extensions and cladding in these areas need planning permission. Check the council's constraints map for your address before relying on permitted development. Source
- Article 4 directions. Several Merton conservation areas, including Merton Park and parts of Wimbledon, are understood to carry Article 4 directions removing permitted development for front alterations, windows and boundary treatments. The exact rights removed vary by area, so we confirm with the council before recommending a route.
- River Wandle flood zones. The Wandle runs through Mitcham, Colliers Wood, Merton Abbey and Wandsworth-bound Earlsfield. Land along it falls within Flood Zones 2 and 3 and some areas also have surface water flooding history. Basements and new ground-floor habitable rooms may need a flood risk assessment. Source
- Metropolitan Open Land and Green Belt. Wimbledon Common, Wimbledon Park, Mitcham Common, Morden Hall Park and Cannizaro Park are Metropolitan Open Land, and the borough's south-western edge includes Green Belt. Houses backing onto these areas are judged on the effect of extensions and outbuildings on openness.
- Listed buildings and Wimbledon Village character. Wimbledon Village, Merton Park and Mitcham Cricket Green contain many listed buildings and locally listed houses. Listed houses need listed building consent for almost any alteration; locally listed houses attract closer design scrutiny.
What Merton planning officers look for
Merton's officers follow the London-wide conventions for householder extensions, and refusals most often cite excessive depth beyond neighbouring precedent, breach of the 45-degree rule or bulky roof extensions. In Wimbledon Village and the Merton Park garden suburb expect detailed comments on materials, windows and boundary treatments, and resistance to front dormers. On 1930s streets in Morden and Mitcham side extensions are expected to be set back and lower than the main roof so the gap between semis is kept. 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.
