The short answer
Often not, but it depends on the extension, not the flat roof. A single-storey extension can fall under permitted development, which sets limits on size, height and position, and those rights do not apply to flats and are narrower for listed buildings, conservation areas and homes under an Article 4 direction. The flat roof covering itself rarely needs consent, though Building Regulations apply. Speak to your council's planners before you start.
Where permitted development stops, what can remove it, and which rules still apply to the roof of an extension that needs no planning application.
Permitted development for a single-storey extension
Planning permission is a question about the building work, not about the flat roof on top of it. Many single-storey extensions to a house can be built under permitted development, which means national rules already grant the permission and no application is made. Those rules set limits on how far the extension reaches, how high it stands, how close it sits to a boundary and what it is built from.
The precise limits are published by the Planning Portal, and they change from time to time, so check the current figures there before you rely on a drawing. A flat roof usually helps an extension stay low, which can be useful against a height limit, but the limits apply to the whole extension.
A flat roof extension that goes beyond the limits needs a householder planning application. So does one that departs from the conditions on materials or position. If your plans are close to a limit, ask the council's planning team before the builder starts, because getting it wrong can mean altering or removing finished work.
Our answer on Building Regulations for flat roofs covers the separate approval that the roof construction may need.
When permitted development rights do not apply
Several situations narrow or remove those national rights. The Town and Country Planning (General Permitted Development) Order 2015 defines a dwellinghouse as a house, and it "does not include a building containing one or more flats, or a flat contained within such a building". So flats and maisonettes cannot rely on house rights, and an application is the safer starting point.
An Article 4 direction lets a council take away chosen permitted development rights within a mapped area, as GOV.UK explains in its guidance on when permission is required. Ask your council whether one covers your street, because the direction's wording decides what is lost.
Designated land adds more care. A conservation area, a National Park and a National Landscape all count as article 2(3) land under the Order, and the rules for some alterations are tighter there. Check with the council's planning team, or the South Downs National Park Authority if you are inside the Park, before you plan around a limit.
Previous extensions and the age of the house can also use up allowances. Planning officers can tell you what counts as the original house.
Listed buildings and the roof covering
A listed house may still keep permitted development rights, but those rights do not remove the need for listed building consent. Section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 bars any alteration or extension altering a listed building's character, unless the works are authorised. An extension to a listed house will be looked at closely, so talk to the council's conservation officer at the outset.
The roof covering is a different matter. Under section 55 of the Town and Country Planning Act 1990, works which leave the outward look of a building materially unchanged fall outside the definition of development. Renewing a flat roof covering like for like, in the same colour and form, sits on that side of the line, which is why re-covering is rarely a planning matter.
A new roof lantern, parapet, raised edge or change of colour can alter the appearance enough to be a different question. If you want one, ask the planning team before ordering. We would rather you ask first than find a problem after the work is finished.
Our flat roof balcony answer covers another change that raises its own questions.
Building Regulations still apply to the roof
Planning and Building Regulations are separate systems. An extension that needs no planning application may still need Building Regulations approval, because that approval covers structure, drainage, fire safety and energy performance. A new roof on an extension is part of that. Talk to your building control body at the start, whether that is the council's service or a registered building control approver.
For a new roof as part of an extension, Approved Document L 2021 sets a maximum U-value of 0.15 for the roof, in Table 4.2, in force until 24 March 2027, when the 2026 edition takes over. Your designer or building control body will say which edition applies to your start date.
Replacing the covering on an existing flat roof is a different case. By the Planning Portal's test, no approval is normally required while under 25 per cent of the building envelope and under 50 per cent of the roof is touched. Cases needing approval regardless include a covering weighing 15 per cent or more above the old, and a covering whose fire performance differs markedly. Approved Document L also classes a renewed flat roof membrane as renovation.
See the flat roofing page for the full range of coverings.
More on this in Flat Roofing in Crawley
Two more extension questions
Do I need permission to re-cover an existing flat roof?
Usually not for planning, if the new covering matches the old in appearance, because a like-for-like re-cover does not change the look of the building enough to count as development. Building Regulations may still come into it, depending on the proportion of roof being redone and the covering chosen. Put the question to building control before work starts.
Who confirms my extension needs no planning application?
Your local planners can say whether permitted development covers your plans. Ask for the reply by email and file it with the deeds and surveys for the house. If you later sell, a buyer's solicitor may ask what was checked before the extension went up, and a written reply answers that.