Do I need planning permission for a loft conversion?

Permitted development is planning permission granted nationally for small works to houses, provided every condition is met. For lofts it sits in the General Permitted Development Order 2015, Schedule 2, Part 1: Class B covers additions that change the roof's shape, such as dormers and hip-to-gable conversions, and Class C covers rooflights. Miss one condition and the whole addition falls outside permitted development.

Everything is measured against the original house, so earlier roof work counts against the limits.

The allowance is the extra volume the new roof encloses compared with the old one, generally measured from the outside. It is 40 m³ on a terraced or end-terrace house and 50 m³ on a semi or detached house (B.1(d)).

A rear dormer uses it in proportion to its width, depth and height. A small dormer over the stair uses little; a full-width box dormer across a London terrace uses most of it and can go over on a deep house or a tall roof. Any earlier roof addition comes off the same allowance, so calculate the volume from measured drawings before relying on it.

It must sit at least 20 cm back from the original eaves, measured up the slope (B.2(b)(i)), must not overhang the outside walls (B.2(b)(ii)) and must stay below the existing ridge (B.1(b)). The materials must look similar to the existing house (B.2(a)); a dormer clad in a contrasting material may need an application.

Class B also rules out verandas, balconies and raised platforms, so a roof terrace or a balcony off the new room needs a planning application. Any side-facing window must be obscure-glazed and fixed shut below 1.7 m above the floor (B.2(c)).

Often, on the same terms. A hip-to-gable conversion builds the sloping side roof up into a vertical gable and is exempt from the eaves set-back. It still counts towards the volume allowance, usually alongside a rear dormer, so the two are added together. On a corner plot the side slope may front a highway, which brings in B.1(c).

An L-shaped dormer adds a second dormer over the back addition of a terraced house. Both count against the one 40 m³ allowance, and this is where terraced lofts most often run out of volume.

In practice, yes. A mansard replaces the back slope with a near-vertical wall and a flat top, which usually takes the roof past the volume allowance and changes its shape and height in ways Class B was not written for. Treat a mansard as needing a householder planning application, at £548. Some London councils accept mansards along terraces that already have them and publish design guidance; others resist them, so read the council's guidance before paying for drawings.

In a conservation area, Class B does not apply at all (B.1(f)). Any dormer or hip-to-gable there needs a planning application, with close attention to design and materials. Rooflights are still normally permitted development under Class C, subject to the limits checked in the report.

An Article 4 direction is a council order withdrawing chosen permitted development rights in a defined area. Some remove Class B or Class C; many concern something else, such as converting houses into shared homes. The direction's text says which. Open data records them for only about half of London's boroughs, so check with the council.

No. Householder permitted development applies to houses only, so a loft conversion in a top-floor flat or maisonette always needs a planning application, at £548. In a leasehold flat the roof space usually belongs to the freeholder, so you will need their written consent as well, and often a change to the lease.

No, it is a separate legal duty under the Party Wall etc. Act 1996, and it applies whether or not the loft needs planning permission. Most terraced and semi-detached lofts put steels into the party wall or cut into it, which needs a party structure notice served on each neighbour at least two months before work starts. Written consent costs nothing; dissent, or no reply, brings in surveyors and an award, which you pay for.

Yes, for almost any permitted development loft. A certificate of proposed lawful use under section 192 is the council's formal decision that the planned work is permitted development. The fee is £274, plus £85 if you apply through the Planning Portal, and the council should decide within 8 weeks. It confirms your reading of the rules before the roof comes off, and a buyer's solicitor will ask for it when you sell.

For a loft conversion with a rear dormer on a two-storey mid-terrace house, with no address so no register read, the report's planning screen gives the route as "Likely permitted development (no planning application)", which means normally permitted development, subject to the limits checked in the report. Marked to measure or confirm: conservation area (planning.data.gov.uk); roof volume allowance (B.1(d)); not on the front (B.1(c)); set back from the eaves (B.1(b), B.2(b)); side windows (B.2(c)).

For your own address it reads the conservation area and Article 4 registers, applies the allowance for your type of house, and switches to an application for a mansard or a flat. It is a screen, not a decision: the council decides.

Loft conversion costs, the planning route and a timeline in each London borough.

Barking and Dagenham · Barnet · Bexley · Brent · Bromley · Camden · Croydon · Ealing · Enfield · Greenwich · Hackney · Hammersmith and Fulham · Haringey · Harrow · Havering · Hillingdon · Hounslow · Islington · Kensington and Chelsea · Kingston upon Thames · Lambeth · Lewisham · Merton · Newham · Redbridge · Richmond upon Thames · Southwark · Sutton · Tower Hamlets · Waltham Forest · Wandsworth · Westminster

Do rooflights in a loft need planning permission?

Rooflights are normally permitted development under Class C, subject to the limits checked in the report: they must project no more than 0.15 m from the slope and sit no higher than the highest part of the roof (C.1(b), C.1(c)). That holds in a conservation area too, unless an Article 4 direction removes it, so check with the council.

Can I start a permitted development loft without telling the council?

Not safely. A lawful development certificate at £274 confirms the planning position in writing. Building regulations approval is needed either way, through the council or a registered building control approver, at about £1,050 to £1,700 for a loft.

What happens if my loft turns out not to be permitted development?

The council can ask for a retrospective planning application at £548, and if it refuses, it can require the work to be altered or removed. A sale can also stall without a certificate. Measuring first, and a certificate where the limits are close, avoid both.