Can I add a mansard if one house on my terrace already has a roof extension?

Not necessarily. Policy CD12 resists additional storeys on terraces that are already broken only by isolated roof additions. It supports infilling where the character of the terrace has been severely compromised by a variety of roof extensions and infill would help to reunite the group. One neighbouring extension is not a precedent on its own, so we count and map the roofs along the terrace before quoting.

Where do I find what my conservation area allows on roofs?

In the Conservation Area Management Plan for your area, which the Local Plan says includes specific guidance on roof changes that may be accepted and those that would be resisted. The council's Article 4 page lists the streets covered by a direction, and each entry links to the direction itself. Check both before you commission drawings.

Can I add a storey to a mansion block in Kensington and Chelsea?

Policy CD12 resists it where the extra storey would add significantly to the bulk of the block or unbalance its architectural composition. The Local Plan's supporting text says its Character Study identifies the building types, such as linear blocks and other flat-roofed buildings, where roof alterations may be suitable.

How is the 40 m³ or 50 m³ measured?

It is the volume added beyond the original roof slope, including any earlier roof extensions, not the floor area of the new room. We calculate it on the drawings and include the calculation in the Lawful Development Certificate application.

Can I have a front dormer?

Not under permitted development: nothing may extend beyond the plane of the roof slope facing a highway. Front rooflights are fine. A front dormer needs planning permission and most councils refuse them unless the street already has them.

Do I need Building Regulations approval as well?

Yes, always. Permitted development and planning permission concern what you may build; Building Regulations govern how it is built, including the new floor structure, the stair, fire escape and insulation. We prepare that package after the planning route is confirmed.

How long does Kensington and Chelsea take to decide?

The statutory period is 8 weeks from validation for both a householder application and a Lawful Development Certificate, and 42 days for a larger home extension prior approval. Busy periods can push decisions past the statutory date; we tell you where the council is running.

Loft Conversions in Kensington and Chelsea

A loft project in Kensington and Chelsea starts with the roofline, not the room underneath. The stucco and brick terraces of Kensington, Chelsea and Notting Hill, the Victorian and Edwardian mansion blocks of Earl's Court and the King's Road, and the artisan cottages of Chelsea and North Kensington all present a roof that is read as part of a street or group. The usual project is a mansard or roof extension where the terrace pattern allows one, and the question that decides it is whether the neighbouring roofs already break the line or still form an unbroken run.

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 Kensington and Chelsea

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 Kensington and Chelsea are listed below, taken from our Kensington and Chelsea area page. Each one is a reason to confirm the address on Kensington and Chelsea's constraints map before relying on permitted development.

Conservation areas. Conservation areas cover about three quarters of Kensington and Chelsea, so almost every house in the borough is inside one. Source
Basement Article 4 direction and basement policy. The council has an Article 4 direction removing permitted development for basement excavations across the borough, and its Local Plan basement policy limits new basements to a single storey beneath the house and no more than half the garden, with structural, drainage, construction traffic and neighbour-impact information required. Source
Listed buildings. Kensington and Chelsea has thousands of listed buildings, including whole terraces and squares in Kensington, Chelsea and Notting Hill.
Article 4 directions on external alterations. Several conservation areas carry Article 4 directions removing permitted development for front alterations, painting, windows, roofs and boundary treatments.
Thames flood zone. Chelsea Embankment, Cheyne Walk and the riverside streets fall within Flood Zones 2 and 3, protected by the Thames Barrier and river walls. Source

How Policy CD12 treats a roof extension in Kensington and Chelsea

The Local Plan (adopted July 2024) deals with roof extensions in Policy CD12, which covers roof alterations and additional storeys. It requires them to be sympathetic to the form and character of the building or group of buildings. It permits well-designed additional storeys and roof-level alterations where they make a positive contribution to the townscape, including where a terrace or group has been severely compromised by a variety of roof extensions and infilling between them would help to reunite it.

The same policy lists where the council will resist an additional storey. That includes a complete terrace or group whose roof line is unimpaired by extensions, even where the proposal is a coordinated design for the whole terrace, and buildings or terraces that already have an additional storey or mansard. It also covers buildings with a roof structure or form of historic or architectural interest, terraces that are already broken only by isolated roof additions, and pitched roofs with eaves. Mansion blocks are listed too, where an additional storey would add significantly to the bulk or unbalance the architectural composition.

Dormers get their own line: the policy resists alterations to dormer windows of architectural and historical interest, even where others on the same terrace have been altered. The supporting text adds that roof additions can change the street, the skyline seen from neighbouring houses and streets, and daylight and sunlight to neighbours, and that proposals must not individually or cumulatively dominate the original building or its townscape.

Conservation areas, management plans and Article 4 directions on roofs

The Local Plan says about three quarters of the borough is within conservation areas and that it contains over 4,000 listed buildings, so most roof proposals are judged against that setting. Its roof extension policy notes that Conservation Area Management Plans include specific guidance on the changes to roofs which may be accepted and those which would be resisted, and that Neighbourhood Plan policies also carry locally specific guidance on roof extensions. We read the plan for your street alongside CD12 before we advise on a design.

The council also runs Article 4 directions in conservation areas, controlling small-scale changes to the outside of houses. Its page lists changing windows, front doors, boundary walls and roof coverings among them, set out street by street with a start date for each. The page does not say which direction covers which works, so we open the direction for your address to see whether it reaches the roof before we recommend a route.

What Kensington and Chelsea planning officers look for

Officers expect extensions to be subordinate, to follow the established pattern on the terrace and to use matching materials and traditional detailing; rear extensions that rise above the closet wing, roof extensions on terraces with unbroken rooflines and any front-elevation change are commonly refused. Pre-application advice is available for a fee and we recommend it for almost every project in the borough.

Fees and timelines

Kensington and Chelsea 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

Royal Borough of Kensington and Chelsea: Local Plan 2024 (adopted July 2024)
Royal Borough of Kensington and Chelsea: Article 4 directions in conservation areas
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.
Royal Borough of Kensington and Chelsea: planning: the council's own constraints map, validation checklist and pre-application service.