The permitted development rules for rear extensions
On a house, a single-storey rear extension is permitted development if it projects no more than 3 m from the original rear wall on a terraced or semi-detached house, or 4 m on a detached house, is no higher than 4 m, has eaves no higher than 3 m where it comes within 2 m of a boundary, and uses materials similar in appearance to the house. The larger home extension route allows 6 m or 8 m through a 42-day prior approval, where the council consults the neighbours.
Flats and maisonettes have no householder permitted development rights, so any rear extension to a flat needs planning permission. Previous extensions count against the allowance, and the larger home extension route is not available in conservation areas.
What changes in Westminster
In a conservation area a single-storey rear extension can still be permitted development within the 3 m or 4 m limit, but cladding it in materials that differ from the house is not, and the 6 m and 8 m prior approval route is closed.
The local constraints that most often decide the route in Westminster are listed below, taken from our Westminster area page. Each one is a reason to confirm the address on Westminster's constraints map before relying on permitted development.
- Conservation areas. More than 50 conservation areas cover the great majority of Westminster's housing, including Pimlico, Belgravia, Bayswater, Maida Vale, St John's Wood and Queen's Park. Roof extensions, cladding and most external alterations need planning permission rather than permitted development. The council publishes conservation area audits that officers use to judge design. Source
- Basement Article 4 direction and basement policy. Westminster has an Article 4 direction removing permitted development for basement excavations, and its City Plan basement policy limits new basements to a single storey beneath the house and no more than half the garden, with structural, drainage and construction management information required. Basements are common in Pimlico, Belgravia and Maida Vale but tightly controlled. Source
- Listed buildings. Westminster has more listed buildings than any other London borough, including whole terraces and squares in Belgravia, Pimlico, Mayfair and Marylebone. Listed houses need listed building consent for almost any alteration, internal or external, and extensions to neighbours are judged on their effect on the setting of the listed terrace.
- Class MA and other Article 4 directions. A Class MA Article 4 direction removes the commercial-to-residential prior approval route across the Central Activities Zone, and several conservation areas carry directions removing permitted development for front alterations. Coverage differs by area, so we check the schedule for your address before recommending any permitted development route.
- Thames flood zone. Pimlico, Millbank, Victoria Embankment and the riverside fall within Flood Zones 2 and 3, protected by the Thames Barrier and river walls. Basements there need a flood risk assessment and may need flood-resilient design. Source
What Westminster planning officers look for
Westminster is one of the two strictest householder planning authorities in London alongside Kensington and Chelsea. Officers expect extensions to be subordinate, to follow the established pattern on the terrace and to use matching materials; rear extensions above closet-wing height, roof extensions on terraces with unbroken rooflines and front-elevation changes are commonly refused. Basements are decided as much on construction method and neighbour impact as on design. Pre-application advice is available for a fee and we recommend it for most projects in the City.
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.
