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 Westminster
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 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.
