The permitted development rules for two-storey extensions
A two-storey rear extension is permitted development only within tight limits: no more than 3 m deep, at least 7 m from the rear boundary, no higher than the existing house, eaves matching the existing eaves, materials similar in appearance, and any upper-floor side window obscure-glazed and non-opening below 1.7 m.
Most two-storey extensions exceed one of those limits, most often the 7 m rear boundary rule on London plots, so the usual route is a householder planning application. Flats have no permitted development rights.
What changes in Merton
Extensions of more than one storey are not permitted development in a conservation area, so planning permission is always needed there.
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.
