Loft conversions in London: types, permitted development and Building Regulations

The main types of loft conversion

• Rooflight (often called Velux) conversion: keeps the existing roof shape and adds roof windows. Least structural work, least extra space.
• Rear dormer: a box-shaped addition to the rear roof slope that creates full-height floor area. Very common on London terraces.
• Hip-to-gable: straightens a sloping (hipped) side roof into a vertical gable wall, usually on semi-detached and end-terrace houses, often combined with a rear dormer.
• Mansard: rebuilds the roof slope as a near-vertical wall with a flat top. The most space, the most structural work, and usually needs planning permission.

For published price ranges for each type, see our loft conversion cost page.

Permitted development: the official limits

Roof enlargements for houses in England fall under Class B of the householder permitted development rules. The government’s technical guidance sets out the limits. In summary, to be permitted development:

• the additional roof space must not exceed the original roof space by more than 40 cubic metres for a terraced house or 50 cubic metres for semi-detached and detached houses, and any previous roof enlargement counts towards this;
• no part of the house can be higher than the highest part of the existing roof;
• a dormer cannot extend beyond the plane of the roof slope on the principal elevation that fronts a highway, so front dormers need planning permission (rooflights on the front can still be permitted under Class C);
• materials must be similar in appearance to the existing house;
• except for hip-to-gable work, the edge of the enlargement should be set back, so far as practicable, at least 0.2 metres from the eaves, measured along the roof slope;
• side-facing windows must be obscure-glazed and non-opening unless the opening part is more than 1.7 metres above the floor;
• verandas, balconies and raised platforms are not permitted development.

The guidance is clear that if there is any doubt, you should ask your local planning authority. To be certain a scheme is lawful, you can apply for a lawful development certificate; the Planning Portal fee schedule sets that fee at half the equivalent full application fee.

When you will need planning permission

• Flats and maisonettes: householder permitted development rights do not cover them, so a loft conversion in a flat needs planning permission (and, if you are a leaseholder, normally the freeholder’s consent under your lease).
• Conservation areas: Class B roof enlargements are not permitted development on article 2(3) land, which includes conservation areas. Much of inner London is covered.
• Article 4 directions: some boroughs have removed roof-alteration rights on particular streets. Check your borough’s Article 4 map.
• Listed buildings: listed building consent is needed for works affecting character, whatever the planning position.
• Mansards and front dormers, and anything over the volume limits.

GOV.UK’s When is permission required? guidance gives the wider picture.

Building Regulations always apply

Turning a loft into a habitable room needs Building Regulations approval even when planning permission does not. GOV.UK explains how to apply. The main technical areas are:

• Structure: new floor joists and usually steel beams, designed by a structural engineer.
• Fire safety and escape: Approved Document B covers means of escape when a new storey is added, usually a protected route down through the house and suitable smoke alarms. This often affects doors on the floors below.
• Stairs: Approved Document K normally asks for 2 metres of headroom over a stair, but for loft conversions where that cannot be achieved, Diagram 1.4 accepts reduced headroom of 1.9 metres at the centre of the stair, falling to 1.8 metres at the side.
• Insulation and ventilation: Approved Documents L and F apply to the new roof, walls and rooms.
• Electrics and plumbing: a new en-suite or new circuits bring in Part P and water regulations; ask who will certify them.

At the end you should receive a completion certificate. You will normally need it when you sell.

Party Wall and neighbours

On a London terrace or semi, a loft conversion often involves cutting into the party wall to take steel beams, or raising it. That is notifiable work under the Party Wall etc Act 1996. GOV.UK explains the notices and timescales. Serve notice early: it is one of the most common causes of delayed starts.

Choosing a loft conversion company

The same checks apply as for any extension builder: Companies House, insurance, TrustMark or trade association claims checked on the scheme’s own site, recent similar work, a written contract and staged payments. Ask specifically who designs the steelwork, who handles the Building Regulations application and Party Wall notices, and how the roof will be kept watertight while it is open. Our page on choosing an extension builder goes through it step by step.

If you would like us to pass a loft conversion enquiry to an independent loft or extension builder covering your area, use the enquiry form and choose “Loft conversion”. We don’t carry out the work ourselves.

Questions people ask

Related pages

• Extension builders in London
  How to shortlist, check and compare extension builders in London before you sign anything.
• Kitchen extension cost
  Published 2026 figures for kitchen extension costs, what quotes usually include and leave out, and the London premium.
• Loft conversion cost
  2026 published loft conversion costs by type and per m², the cost breakdown, hidden extras and the London premium.

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