Side extensions in Merton depend on the house. Typical homes here: Victorian terraces in Colliers Wood, South Wimbledon, Mitcham and around Haydons Road; Edwardian houses in Wimbledon Park, Raynes Park and West Wimbledon. Designations to check first: Conservation areas; Article 4 directions; River Wandle flood zones. Applications are decided by London Borough of Merton and the Merton Local Plan.
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 Merton
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 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. 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.
- River Wandle flood zones. The Wandle runs through Mitcham, Colliers Wood, Merton Abbey and Wandsworth-bound Earlsfield. 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.
- Listed buildings and Wimbledon Village character. Wimbledon Village, Merton Park and Mitcham Cricket Green contain many listed buildings and locally listed houses.
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. 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
Merton decides a householder application or a Lawful Development Certificate within 8 weeks of validation. The council fee is £528 for a householder application and £264 for a Lawful Development Certificate (England, from 1 April 2025; verify before applying).
Our own fee is fixed once quoted. The quote tool gives an indicative figure in minutes and a written quote follows within 24 hours.
Primary sources
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2: the permitted development limits quoted above.
- Planning Portal: do you need permission: the national guidance and the application service every council accepts.
- London Borough of Merton: planning: the council's own constraints map, validation checklist and pre-application service.
