People usually come to this after a flat roof has failed for the second or third time. A pitched roof sheds water instead of holding it, and it can open up the space below. Those are good reasons to consider the change. They are also the reasons the work attracts attention from the council, because a flat roof turning into a pitched one alters what the building looks like from the street and what it weighs on its walls.
Two separate systems apply, and they ask different questions. Planning asks whether the new shape is acceptable: how high it rises, how far it projects and where the house stands. Building Regulations ask whether the result is safe and efficient: whether the structure carries it, whether it keeps heat in and whether the roof space can breathe. Passing one does not settle the other.
This guide takes the planning side first, then the Building Regulations, then the practical parts of the build. It is general information, not legal advice. For any particular house, put the question to the planning team before work is priced, because a few metres of ridge height or a conservation area boundary can change the answer. The wider range of this work is described on our pitched roofing page.
Why a flat-to-pitched conversion is rarely a like-for-like job
Planning rules are relaxed for repairs that leave a building looking the same. The source is section 55(2)(a) of the Town and Country Planning Act 1990: maintenance, improvement or alteration that does not materially affect the building's external appearance is not classed as development. A fresh membrane on the existing flat roof falls comfortably inside that wording, because the house looks the same afterwards. That is why a straightforward flat roof renewal rarely raises any planning question at all.
A pitched roof is another matter. It swaps a horizontal line for a sloping one, adds a ridge, often changes the height of the eaves line and brings in new materials. All of that is visible from the road, so the allowance for straight replacement does not carry over, and the question becomes whether the work is permitted development or needs a full application.
The usual starting point
For a house, the starting point is the General Permitted Development Order 2015, Schedule 2, Part 1. It grants a set of rights to alter a dwellinghouse without a planning application, subject to limits. Class C deals with any other alteration to a house roof, which makes it the nearest category for a reshaped one. Larger roof enlargements are handled in a separate class, so the first practical step is to ask the planning team which class, if any, your design falls into.
Flats are different again. The Order's own definition of a dwellinghouse leaves out blocks of flats and individual flats, so none of the Part 1 rights reach them. For a flat-roofed block, planning permission is the route.
The Class C limits a pitched roof tends to break
Class C is written around small alterations: a roof window, a flush rooflight, a replacement of one covering with another. It sets limits that a pitched conversion can overshoot quite easily. Under C.1(b), it must not stand more than 0.15 metres proud of the face of the old slope, measured at right angles to that surface. Under C.1(c), the alteration may not finish above the roof's existing high point.
Both tests are measured against the roof that exists now. A flat roof has one level, and a pitched roof built over it rises to a ridge above that level. The ridge is by definition higher than anything on the roof it replaces, so on a plain reading the conversion fails the height test at the first step. Where the pitch is hidden behind a parapet and does not exceed the old high point, the position can be different, and that is the kind of detail only the council can confirm.
Class C also excludes some items outright. Chimneys, flues and soil and vent pipes are outside its protection, and so is solar equipment.
Roof additions under Class B
Where a conversion adds volume, Class B for roof additions comes into play. Its limits are tighter: under B.1 the addition cannot rise higher than the present ridge, cannot project past a front slope facing the road, and is capped at 40 cubic metres of extra volume on a terrace or 50 cubic metres in other houses. Class B is never available on article 2(3) land, which the next section explains.
Conservation areas, the National Park and Article 4 directions
Where a house stands matters as much as what is built. Article 2(3) land takes in conservation areas and National Parks, along with the Broads, World Heritage Sites and designated areas of outstanding natural beauty. On that land, Class B roof additions are not permitted development. A pitched conversion that counts as a roof addition therefore needs a planning application inside a conservation area or the South Downs National Park, however modest the design.
Class C has no article 2(3) exclusion, so it can still apply in those places, unless an Article 4 direction or a planning condition removes it, or the building is listed. Several local directions do remove it. Two Adur directions of 2023, covering Old Shoreham and Kingston Buci, take away Class C for houses whose fronts face a highway or public path, roof coverings included. Worthing has its own. Warwick Gardens withdraws the right to alter the appearance of a roof slope, or the material it is made of, on road and open-space frontages, Warwick Place removes Class C on the frontage to that street, and Western Row is marked as having every dwellinghouse permitted development right removed.
Other directions exist, and some Worthing ones, such as Ambrose Place and Cowper Road, do not touch roofs. A direction is no bar to the work; it simply means permission must be sought for the matters it lists.
Listed buildings and the Park
Listed building consent is a separate requirement. Any work that affects a listed building's special character needs consent first, and proceeding without it is a criminal offence. A house within the South Downs National Park has the Park's own authority as its planning authority, so address that question to the Authority.
What Building Regulations ask of a new pitched roof
Building Regulations approval applies to a conversion even when no planning permission is needed. By the Planning Portal's list, approval is needed for structural alterations, for a covering that would behave differently in a fire, and for a heavier covering, specifically one that adds 15 per cent to the roof's weight or beyond. A conversion almost always involves structural alterations, since the old flat roof structure was designed to carry a different load in a different way.
Approved Document A phrases its weight test as an increase of more than 15 per cent, and treats any strengthening needed to cope as a material alteration. The two wordings differ by a hair, and the practical advice is identical: ask a structural engineer or surveyor to inspect before anything is ordered.
Heat loss and insulation
Approved Document L treats stripping and renewing a flat roof's waterproofing as renovating a thermal element; if more than half the surface is renovated, the entire element has to reach the current standard. A new roof element in an existing dwelling has a limiting U-value of 0.15 W/(m²·K) in Table 4.2, so the new roof must be insulated to a high standard, which affects the depth of the build-up.
Who approves it
The approval comes from a registered building control approver or from the council's building control team. Adur & Worthing share one team, based at Worthing Town Hall. The councils say the owner has a duty to inform building control as particular stages are reached on site, and that the team can no longer offer design advice. For notifiable work we explain the consent and deal with the notification, so the stages are not left for you to remember.
Ventilation, underlay and covering for the new pitch
Once the structure and the paperwork are settled, the new roof has to work as a roof. Three choices matter most: the covering, the underlay and the ventilation. They are linked, because the pitch you can afford to build limits which coverings are allowed.
A shallow pitch is cheaper to build but narrows the choice of tile. Marley lists its Lincoln clay pantile at a minimum of 15 degrees, while its clay plain tiles start at 30 degrees and rise to 40. Our guide to matching old roof tiles explains how a new pitch affects what will blend with the neighbours. Marley adds that under 30 degrees, the face of the roof into the wind can see severe suction, which our guide to tile fixing in coastal wind explains in full.
Underlay and airflow
A cold pitched roof needs air moving through it. Marley's summary of BS 5250 gives a continuous gap of 10 mm along the eaves for pitches above 15 degrees with a high-resistance underlay, rising to 25 mm at 15 degrees or less, and an added 5 mm at the top of the roof once the pitch passes 35 degrees. Condensation that forms in a poorly vented roof is a common reason a new roof starts to smell damp within a couple of winters. The full detail is in our guide to roof underlay and ventilation.
If the new roof space is going to become a room, further rules apply. A habitable loft above two storeys also needs a protected stairway for escape, since the Approved Document B rule is built around a 4.5 metre height above ground.
Costs, quotes and what to settle before work starts
A flat-to-pitched conversion cannot be priced responsibly without a survey, because the structure, the pitch and the access decide the figure. For orientation only, MyBuilder's guide to a new roof (updated 21/08/2026) gives typical re-roof ranges by shape on a semi-detached house: £2,500 to £6,000 for a flat roof, £5,000 to £8,000 for a gable roof and £8,000 to £12,000 for a hipped one. Those are re-roof figures, not conversion prices, and a conversion adds the frame that carries the new roof. The same guide puts scaffolding at around £1,125.
With two or more quotes in hand, see whether VAT and scaffolding appear in each figure, since either can be missing. A conversion may also bring fees for building control, and the councils publish theirs, with VAT on all fees except regularisation fees.
Before you commission anything
Settle four things. Ask the planning team whether the design needs an application, and whether your house lies in a conservation area, the National Park or an Article 4 direction. Ask an engineer or surveyor to check what the walls can carry. Decide the covering, because it fixes the minimum pitch. And agree who handles the building control application.
We begin with a survey of the existing flat roof, the walls and the loft or ceiling void, with photographs of your own roof. A written quote comes after that, giving a single total, with the structure, the new covering and the work to the walls all described. A conversion is not always the smallest job that solves the problem, and a well-built replacement flat roof is sometimes the better recommendation.
