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 Lambeth
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 Lambeth are listed below, taken from our Lambeth area page. Each one is a reason to confirm the address on Lambeth's constraints map before relying on permitted development.
- Conservation areas. Lambeth has numerous conservation areas covering much of Kennington, Stockwell, Clapham, Brixton, Herne Hill and Streatham. Roof extensions and cladding in these areas need planning permission rather than permitted development. Check the council's constraints map for your address before assuming PD applies. Source
- Article 4 directions. Lambeth has Article 4 directions in a number of conservation areas removing permitted development for front alterations and boundary treatments, and a Class MA Article 4 covering Waterloo, Vauxhall and other central and town-centre locations that removes the commercial-to-residential prior approval route. We confirm coverage for your address with the council.
- Thames flood zones. Waterloo, Vauxhall, Nine Elms and the riverside at Lambeth fall within Flood Zones 2 and 3, protected by the Thames Barrier and river walls. Basements and new ground-floor habitable rooms there may need a flood risk assessment; surface water flooding is also a recorded issue in parts of Herne Hill and Streatham. Source
- Listed buildings and the Kennington and Clapham set pieces. Kennington, Clapham Old Town, Stockwell and the Vauxhall riverside contain many listed Georgian and Victorian houses and terraces. Listed houses need listed building consent for almost any alteration, and the setting of listed terraces is considered for extensions to neighbours.
- Metropolitan Open Land. Clapham Common, Brockwell Park, Streatham Common and Norwood Park are Metropolitan Open Land. Houses fronting or backing onto them are judged on the effect of extensions and outbuildings on openness and setting.
What Lambeth planning officers look for
Lambeth's officers apply the Local Plan's residential design and amenity policies fairly predictably. Rear extensions deeper than neighbouring precedent, side returns that breach the 45-degree rule to the neighbour's rear window and roof extensions in conservation areas are the common refusal reasons. Mansards are accepted on terraces where the street already has them; front dormers and roof extensions on butterfly-roofed terraces attract resistance. Pre-application advice is available for a fee, and Lambeth's decision times can stretch beyond the statutory period at busy times.
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.
