Do you need planning permission for a loft conversion in London?

Permitted development is the default for houses, not flats

Planning permission for a loft conversion is not something most London homeowners have to apply for, because the Government has already granted it. Schedule 2, Part 1, Class B of the General Permitted Development Order 2015 gives every house a planning permission for the enlargement of a dwellinghouse consisting of an addition or alteration to its roof, subject to limits. Stay inside the limits and there is nothing to apply for. Step outside them, or lose the right altogether, and you need a householder planning application from the borough.

The right belongs to houses. The Planning Portal lists the houses that cannot use it: a building containing one or more flats, or a flat in such a building; a house that has already had storeys added under permitted development; a house created from a non-residential building under permitted development; a house built as a new dwelling under permitted development; and any house on what the Order calls article 2(3) land, which includes every conservation area. A planning condition on an earlier permission can also have removed the right, which is common on newer estates, so the title deeds and the original permission are worth reading before the drawings are commissioned.

The permitted development limits for a loft conversion

These are the conditions in Class B and, for rooflights, Class C, as the legislation stands in September 2026. The Government's technical guidance for householders explains how each one is measured.

How the 40 and 50 cubic metre allowance really works

The allowance is measured against the original roof space: the roof as the house was first built, or as it stood on 1 July 1948 if it is older than that. Every enlargement to the roof since then counts against it, whether or not it was permitted development at the time, because Class B.3 defines the resulting roof space as the roof space as enlarged, taking into account any enlargement to the original roof space. A previous owner's small dormer at the back therefore eats into what you can add now.

A terrace house, for this purpose, is a house in a row of three or more that shares a party wall or has a main wall adjoining the house on either side, and an end-of-terrace house counts as a terrace house because it adjoins one that does. A semi-detached house gets 50 cubic metres. On a typical London terrace a full-width rear dormer uses most of the 40, and an L-shaped dormer that also runs over the back addition uses more, which is why L-shaped dormers in London often go through a planning application rather than permitted development. A hip-to-gable conversion on a semi adds the volume of the new gable before the dormer is counted, so the combination can exceed 50 cubic metres on a larger roof. Measure before you assume.

When a loft conversion needs a planning application

• A dormer on a roof slope that faces the street, because it extends beyond the plane of the principal elevation.
• A mansard, which rebuilds the front slope, and almost always exceeds the volume allowance as well.
• Anything that rises above the existing ridge.
• A roof terrace or a balcony with a platform.
• A dormer that, added to previous enlargements, takes the roof over 40 or 50 cubic metres.
• Any conversion in a conservation area, a national park, an area of outstanding natural beauty, the Broads or a World Heritage Site, other than a rooflight conversion under Class C.
• Any house covered by an Article 4 direction that withdraws Class B, or a house whose permitted development rights were removed by a condition on an earlier permission.
• Every flat and maisonette, because householder permitted development rights do not apply to them.
• A listed building, which needs listed building consent for the works as well as any planning permission.

Conservation areas and Article 4 directions in London

In a conservation area the Order simply switches Class B off: paragraph B.1(f) says development is not permitted if the house is on article 2(3) land. You can still install rooflights under Class C, within its 0.15 metre and height limits, so a Velux conversion of a roof that already has the head height is usually possible without an application, but a dormer of any size needs planning permission. Boroughs then judge the dormer on its design against their conservation area guidance, which generally favours rear dormers that are subordinate to the roof, set in from the edges and clad to match over full-width boxes.

An Article 4 direction is different. It is a direction made by the borough under article 4 of the Order that withdraws specified permitted development rights across a defined area, and the Government's planning practice guidance says it should be applied in a measured and targeted way, to the smallest area possible. Directions are most common in conservation areas but can cover any street, and they are not visible from the pavement. Boroughs publish their own registers or maps. Check yours before anything else, because a dormer that would be permitted development three streets away can need an application on yours.

Flats and maisonettes

The permitted development rights that cover loft conversions on houses do not apply to flats, so a top-floor flat converting its loft needs a full planning application whatever the size of the dormer. There is a second hurdle before planning: the loft is often not part of the flat's lease at all, and even where it is, the lease usually requires the freeholder's consent to structural alterations. Establish who owns the roof space and what the lease allows before you pay for drawings, because a planning permission is worthless without the right to build.

The lawful development certificate: not compulsory, but get one

If your conversion is permitted development you can apply to the borough for a lawful development certificate confirming that the proposed works do not need planning permission. It is not compulsory. It is, however, the document a buyer's solicitor asks for years later, when the only alternative is arguing from old photographs about the volume of a roof that no longer exists. The council has eight weeks to decide, and you can appeal if it refuses or fails to decide.

The fee for a certificate for proposed works is half the householder application fee, £274 in England from April 2026, and a certificate for works already carried out costs the same as a full application, £548 (the householder fee in the Planning Portal's fee guidance), plus the Portal's online service charge, usually £99 including VAT (its Band 1; some application types carry no charge). Apply for the proposed-works certificate before you start: the Government's guidance explains that a certificate for proposed development is presumed lawful unless there is a material change before the development begins, and a new Article 4 direction taking away the relevant right is exactly such a change.

The 10-year rule

People ask about the ten-year rule because they have been told that unauthorised building work becomes immune from planning enforcement after long enough. It does, but the period changed in 2024. Section 171B of the Town and Country Planning Act 1990, as amended by section 115 of the Levelling-up and Regeneration Act 2023, gives the council ten years from substantial completion to take enforcement action against building works completed on or after 25 April 2024. Works substantially completed before that date keep the old four-year limit. The Government's enforcement guidance sets this out, along with the exception: where a breach has been deliberately concealed, the council can apply for a planning enforcement order after the time limit has expired.

It is not a strategy. A buyer's solicitor will ask for the planning paperwork and a mortgage lender may refuse to lend on a house with an unresolved breach, and building control has its own clock: since the Building Safety Act 2022 amended section 36 of the Building Act 1984, a notice requiring work that contravenes the Building Regulations to be altered or removed can be given up to ten years from completion, where it used to be twelve months, for work completed since the change took effect on 1 October 2023; take advice on older work. A conversion that was never inspected can be regularised, but it is inspected retrospectively, with the fire protection and the structure opened up if necessary, and it is far cheaper to do it once.

Building Regulations are separate, and always apply

Permitted development answers the planning question and nothing else. Converting a loft into a room you can live in needs Building Regulations approval every time, because it adds a floor the existing joists were never designed to carry, cuts the roof, and puts bedrooms above the height people can escape from through a window. The regulations cover the strength of the new floor, the stability of the roof, a proper staircase, sound insulation from the rooms below, insulation of the new roof, and fire: on a two-storey house that becomes three storeys, the stairway becomes a protected escape route with fire-resisting doors and mains-powered interlinked smoke alarms on every level, which is the part of a loft conversion that touches rooms nobody planned to touch.

You apply to the borough's building control service or a registered building control approver, they inspect at stages and issue a completion certificate at the end. The four London boroughs that publish a clear schedule charge £859–£1,288 including VAT for a loft conversion of up to 60 square metres (2025/26 and 2026/27 schedules). Our loft conversion cost guide lists the fees borough by borough alongside the build prices.

Party wall notices on a London terrace

The new floor in a loft conversion is usually carried on steel beams that bear into the party walls, and cutting into a party wall to take the bearing of a beam is one of the rights listed in section 2 of the Party Wall etc. Act 1996. That means a party structure notice to each neighbour whose wall you cut into, served at least two months before the work starts. The neighbour has 14 days to consent in writing; if they do not, a dispute is deemed to have arisen and surveyors settle it by an award. Published London figures put an agreed surveyor at £700–£1,800 per adjoining owner, and a case where the neighbour appoints their own surveyor at £1,400–£4,500 per neighbour, which the building owner usually pays. It has nothing to do with planning, and it is the item most often left until it delays the start. Our guide to party wall agreements in London explains the notices in full.

If you do need to apply: fee, timing and what helps

A householder planning application in England costs £548 from 1 April 2026, the figure in the Planning Portal's fee guidance; draft regulations laid before Parliament on 16 July 2026 would raise it to £575 from 8 December 2026, so check the fee before you apply. On top comes the Portal's online service charge, usually £99 including VAT (its Band 1; some application types carry no charge). The statutory period for a decision is eight weeks, though validation delays and requests for an extension of time are common in the busier boroughs, so plan on three months. What helps is a design that answers the objections before they are raised: a rear dormer set in from the party walls and down from the ridge, matching materials, obscure glass to the side, and a set of drawings that shows the neighbours' roofs as well as yours. Pre-application advice is available from most boroughs, usually for a fee, and is worth it in a conservation area.