Replacing a flat roof feels like maintenance, and often it is. Yet the Building Regulations can apply to roofing work, and owners are caught out in both directions: some pay for approval they did not need, and others find at sale that work done years ago has no paperwork behind it.
The rules are not complicated once they are laid out. The Planning Portal gives a pair of tests about how much of the building the work touches. It adds two triggers that bring approval in whatever the size: a heavier covering and a covering that behaves differently in a fire. And separately, Approved Document L, the energy guidance, treats re-covering the waterproofing of a flat roof as renovation, which can mean adding insulation.
This guide sets those points out in order, with the exact wording where it matters, and explains how to ask the right body. None of it is legal advice, and your building control body has the last word on your roof. The guide sits under our flat roofing service, for owners planning a replacement or a new roof.
The Planning Portal's two tests
The Planning Portal, the government's planning and building information service, says no building control application is normally needed for re-roofing when two conditions both hold.
Test one: the building envelope
The first is that "Less than 25 per cent of the total 'building envelope' (walls, floor, roof, windows, door, roof windows and roof-lights) is affected". The envelope is the whole shell of the building, not just the roof. A small flat roof on a large house is a modest slice of it.
Test two: the roof itself
Next comes a second condition: that "Less than 50 per cent of the roof is affected". Here the denominator is the roof of the building, and the test asks how much of it the work covers. Re-covering a porch roof touches only a corner of the roof as a whole. Re-covering the only roof of a single-storey building is all of it.
Both must hold
The two tests are joined by "and", so a job must pass both. A roof can pass the second test and fail the first, or the other way round. Be wary of rules of thumb: none of the councils around Andover sets a percentage figure of its own, so a number you have heard for one council is not a rule.
What "normally" means
Passing both tests points to no application being required in the usual case. That is not a promise that no rule applies, and it does not remove the energy question in a later section.
Weight, fire and the structural check
Two further conditions bring Building Regulations approval in whatever the size of the job.
A heavier covering
The first extra trigger is weight. Approval is needed, the Planning Portal says, where a new material makes the finished covering 15 per cent or more heavier than before. Swapping a light flat roof covering for a heavier one, such as a paved or green finish, is the sort of change that matters. The Portal's advice is to take a structural engineer's or surveyor's view before starting, because a flat roof deck sized for a light membrane may not carry a heavier finish without strengthening.
Fire performance
The other trigger is that "The performance of any new covering will be significantly different to that of the existing covering in the event of a fire". A covering with a different fire rating from the old one needs approval, even if it weighs the same.
The Approved Document A figure
Approved Document A, on structure, calls a significant change in roof loading one where the loading is increased by more than 15%. The two documents use slightly different wording, the Planning Portal saying 15 per cent or more and the Approved Document saying more than 15%, so it is better to quote one and name it than to mix them.
Planning permission is different
A weight increase triggers Building Regulations approval and a structural check. It does not mean you need planning permission, which is a separate system with its own tests.
Part L: a new waterproof layer is renovation
The energy rules sit in Approved Document L. The 2021 edition, with its 2023 amendments, took effect on 15 June 2022 for use in England.
What counts as renovation
Among its examples of renovation of a thermal element, the document names a new waterproof covering laid on a flat roof. Renovation means providing a new layer in the element or replacing an existing one, as Building Regulations 2010 reg 2(1) defines it. So a re-cover of a flat roof is not exempt from the energy rules just because it is like for like.
The thresholds
In Table 4.3 an existing roof is measured against a threshold U-value of 0.35, with 0.16 as the improved value. If a roof is worse than the threshold, the renovation should bring it to 0.16 where that proves practical in technical and functional terms and pays back within 15 years. Paragraph 4.13 describes the fallback: upgrade to the best that is feasible within a 15-year payback, and generally no worse than 0.7.
Common errors
The figure of 0.15 belongs to new roof elements in Table 4.2, not to renovation and not to loft conversions. And Approved Document L 2026, which renumbers the tables as 3.2 and 3.3, takes effect from 24 March 2027, not now.
What it means in practice
In practice, a re-cover may need extra insulation, and the roof edge details and rooflights may need review. Our guide to warm deck against cold deck flat roofs covers the build-ups that achieve these figures.
A new roof on a new structure
A flat roof on a new extension, porch or garage conversion is a different case from re-covering an old one. Here the roof is part of a new element, and the whole building work needs approval.
New elements
Table 4.2 in the 2021 edition of Approved Document L sets 0.15 W/(m²·K) for a new or replacement roof element in an existing dwelling. A roof window must reach a U-value of 1.4, or else reach Band B or better on the Window Energy Rating scale, and a rooflight must reach 2.2. An upstand built on site can have a maximum of 0.35. Those are the figures an inspector expects to see in the insulation design. Our guide to flat roofs on extensions explains the practical details.
Conservatories and porches
The rules for conservatories differ: the Planning Portal's answer on conservatories is that the roof itself must satisfy the rules on structure, energy and ventilation, even if the room as a whole may be exempt. Small porches and similar structures have their own exemptions, and your building control body can say whether they apply.
Garage roofs
A garage roof being replaced like for like is a re-roofing question and follows the tests above. A garage being converted to a room is a change of use, which is a different set of rules again, and the roof is only part of that application.
Structure and drainage
Approved Document C expects roofs to resist rain and to avoid interstitial condensation, and its paragraph 6.8 expects every part of the covering to be fixed appropriately. Those apply to the new roof as to any other.
Asking the right body
The question "do I need approval?" goes to your building control body. There are two kinds.
The council's own team
Local authority building control exists in every area, though some councils share a service. Wiltshire Council runs its own Building Control service on 0300 456 0113. Basingstoke and Deane's Building Control team is on 01256 844844, and Winchester City Council's main number is 01962 840 222. New Forest District Council's team is on 023 8028 5245 and East Hampshire's on 01730 234207. West Berkshire Council works with Wokingham through Building Control Solutions, on 0300 790 0580.
Test Valley
For an Andover address, put the question to local authority building control, or to a registered approver if you prefer.
A registered approver
A registered building control approver is the alternative. That registration regime began on 6 April 2024, and private building control existed before it.
Basingstoke and Deane addresses
For its own addresses, Basingstoke and Deane's building regulations page includes the re-covering of pitched and flat roofs as an alteration project that "might also need" approval. We check before a re-roof there. That is one council's wording, and it gives no percentage.
What to ask
Describe the roof, the covering, the area and the insulation, and ask whether an application is needed and what it will inspect. Ask also how long a decision takes and whether the body wants notice before the deck is covered, because inspections of insulation happen before it disappears under the membrane.
What we do and what you keep
The paperwork around a roof is easier when it is kept as the job goes on, and not rebuilt from memory.
On the survey
We measure the roof, record the existing build-up where we can see it, and photograph the covering, the edges and any rooflights. We note whether the job is likely to affect enough of the building to fall outside the Planning Portal's tests, and whether the weight or fire performance of the new covering differs from the old.
Choosing the route
If an application is needed, we explain what the building control body will want to see, such as the make of insulation, the slope of the deck and the data for the membrane. We confirm the route with the body before work is booked, not after the roof is down.
Keeping the evidence
You receive photographs taken at each stage, from bare deck to finished covering, along with the product data sheets. If you sell, a buyer's solicitor may ask for them, and a completion certificate is the neatest answer where one was issued.
Planning and listed roofs
Class C of the planning rules normally covers a re-roof on a house, but flats have no permitted development rights. A listed building needs listed building consent. Check with Test Valley Borough Council's planning team if yours is one.
Cost
Approval fees and insulation both affect the final figure, which is written down after the survey.
