Homeowners usually ask this question for one of three reasons. The loft is too low to use, a flat or shallow roof keeps leaking, or an extension looks awkward beside the main house and a steeper slope would tidy it up. In each case the idea is the same: take the rafters out, set them at a new angle and put the covering back on.
The pitch is the slope of the roof measured in degrees, and it controls almost everything else about the roof: how high the ridge stands, how far the eaves overhang, which coverings can properly be laid and how much weight the walls carry. Alter it and you are altering the building, not repairing it. This guide explains why that normally takes the job outside permitted development, what the Building Regulations ask of the new structure, and how conservation areas and listed buildings change the picture. This sits inside our wider pitched roofing work. For the related question of how a pitch is measured, see our guide to measuring roof pitch.
Why a new pitch rarely fits within permitted development
Permitted development is the set of rights that let owners of ordinary houses make certain changes without applying for planning permission. Roof work sits in two classes of the General Permitted Development Order 2015. Class B covers enlarging a house by adding to or altering its roof, which is the class dormers fall under. Class C takes in the remaining alterations to a roof, and that is where roof windows sit.
Neither class was written for a change of pitch, and the limits show why. Under Class C, an alteration cannot protrude more than 0.15 metres beyond the plane of the slope of the original roof, and it cannot make the highest part of the roof higher than the highest part of the original. Under Class B, no part of the addition may exceed the height of the highest part of the existing roof, and it cannot extend beyond the plane of an existing slope that fronts a highway.
Now picture steepening a roof. The ridge climbs, because the same span at a steeper angle rises further. That breaks the height limit in both classes. Flatten a roof instead and the plane of the slope has moved, which is the thing the 0.15 metre limit protects. Either way the work usually lands outside the rights.
Both classes are measured against the original roof, so any earlier enlargement by a previous owner can use up part of the allowance before you start.
Whether a particular scheme squeezes inside the limits is for the planning officers to say, at Swindon Borough Council or whichever authority covers your street. Ask them before any timber is ordered.
Raising the ridge, lowering the eaves and the cases in between
Not every pitch change is the same job, and the planning position shifts with what actually moves.
What if only the ridge goes up?
Lifting the ridge by a few courses to gain loft headroom is the classic case. It breaks the rule that nothing may stand higher than the original ridge, so it normally needs a planning application. Neighbours on a terrace notice a raised ridge quickly because it breaks the line shared with the houses either side, and the council sees the street as a whole.
What about changing a hip into a gable?
Swapping a hipped end for a vertical gable changes the shape of the roof and adds volume to the loft. That counts as enlarging the roof space, so the Class B limit applies: no more than 40 cubic metres added in a terraced house, or 50 cubic metres in any other. Anything beyond that, or on land where Class B is withdrawn, needs permission.
Can a shallow roof be made steeper?
Putting a pitched roof where a flat one stood, or lifting a low pitch, raises the highest point too. Our guide to converting a flat roof to a pitched one goes through that route. Whatever the starting point, the test is the same: does the highest part of the roof stay where it was, and does the slope stay in its original plane?
If the answer to either is no, plan for an application. The council will want drawings showing the existing and proposed heights, because the effect on neighbours is what planners judge.
Building Regulations for the rebuilt roof
Even where planning permission is not needed, the Building Regulations are separate and a new pitch almost always triggers them. The Planning Portal lists structural alterations among the reasons a roof job needs building control approval, and cutting out rafters and re-forming a roof is as structural as it gets.
Four things are checked.
- The structure. The new rafters, purlins, ridge beam and wall plates have to carry the covering, snow and wind, and the walls below have to carry the new loads. A structural engineer usually specifies the sizes, as the Planning Portal advises where a roof changes significantly.
- The thermal element. Part L treats a roof stripped to its rafters and then rebuilt as a renovated thermal element. The roof then has to reach the improved U-value in Table 4.3, which is 0.16 W/(m²·K), or 0.15 for a new roof element.
- The covering. A heavier covering that increases the load by 15 per cent or more brings its own approval requirement, and a different fire performance does too.
- Ventilation and means of escape. Cold roof ventilation follows BS 5250, and any loft room created behind the new pitch brings the escape rules into play.
You choose between the council and a registered building control approver, a private option that has existed since 6 April 2024. Swindon Borough Council has its own team, and Wiltshire addresses go to Wiltshire Council's building control team. Notifiable work goes through us: we set out the steps and file the Building Regulations notification.
Conservation areas, National Landscapes and listed buildings
Designated land tightens the rules in layers, and it helps to separate them.
A conservation area, a National Landscape (the name used since 22 November 2023, although the legal designation is still an AONB) and a National Park are all article 2(3) land. On that land Class B does not apply at all, so a roof enlargement of any kind needs an application. Class C survives on article 2(3) land, but a pitch change still trips its height and 0.15 metre limits.
Councils can also remove rights with an Article 4 direction. Swindon's conservation areas page lists 27 areas and notes Article 4 directions for Bishopstone and the Railway Village. In the Railway Village the council's own appraisal describes heavy pitched slate roofs as part of what the designation protects, so a steeper or flatter slope would change the very thing being conserved.
Should the house be listed, any work that affects its special character needs listed building consent, and doing it without consent is a criminal offence. A pitch alteration on a listed roof is about as far from routine as a roof job gets. Even a straight re-cover of a listed roof is worth raising with the conservation officer first.
The North Wessex Downs and Cotswolds National Landscape bodies only advise. The council decides. If your house is near either, it is worth asking the planning team early rather than assuming either way.
What the covering needs once the angle changes
A pitch change does not just alter the outline. It changes which coverings are suitable, and that can decide the whole design.
Each tile and slate comes with a lowest safe pitch set by its maker. Marley's table gives 40 degrees for Canterbury clay plain tiles, 35 degrees for its concrete plain tile and 17.5 degrees for several interlocking concrete tiles. For natural slate, BS 5534:2014+A2:2018 as summarised by Cupa Pizarras puts the minimum at 20 degrees for a 500 mm slate in moderate exposure, laid with a 115 mm headlap. The same source rules out hook fixings on anything under 25 degrees. Lower the pitch below a covering's minimum and rain can be driven or backed up under the laps, so the covering has to change or the pitch has to stay.
Steepening has its own effects. A steeper slope sheds water faster, but on pitches over 35 degrees BS 5250 guidance via Marley adds a continuous 5 mm vent at high level on top of the eaves ventilation. Heavier materials need stronger fixing. Steeper slopes also put more weight on each fixing, so the nailing and clipping specification matters. Our guide to roof underlay explains what goes beneath, and the ventilation guide covers the airflow side.
Lower pitches also sit closer to the edge of what a cheaper underlay can handle. A shallow roof laid with the wrong membrane is a common source of leaks, so read the maker's data sheet before settling on an angle.
Questions to settle before anyone cuts a rafter
The sensible order is to settle the legal route first and the design second, because a refused application wastes the drawings.
Start with the existing roof. Is it the original, has it been altered, and does the house have permitted development rights at all? Rights can be removed by a planning condition or an Article 4 direction, so check which apply to your address.
Then ask what the pitch change is for. If the aim is headroom, a dormer may give more usable floor than a taller ridge, and dormer permitted development rules have their own answer page. If the aim is a roof that stops leaking, a repair or re-cover at the existing angle is usually the smaller job. Our advice is to do whatever small job cures the fault, and you can read how pitched roofs behave in our guide to pitched roof types.
What do I show the council?
Photographs of every elevation, a measured drawing of the roof as it stands, a drawing of the proposed roof and the heights of both ridges above ground. Neighbouring roof heights help too.
Who lives with the finished roofline?
Neighbours, the street and any later buyer. A raised ridge on a semi-detached house can leave a visible step against the other half, and a surveyor acting for a buyer will ask whether the roof was altered and whether approvals exist. Keep the planning decision notice, the building control completion certificate and the structural calculations together with the deeds. Missing paperwork for a roof that was rebuilt years ago is a common snag at sale, and it is far easier to file the documents at the time than to reconstruct them later.
It also helps to talk to the neighbours before the application goes in. Councils invite comments, and a neighbour who has seen the drawings and understood the scaffold, the noise and the dates is less likely to object than one who learns of it from a skip on the drive.
If you book a survey, the Your own roof is photographed at survey and the pictures sit alongside a written quote. Cracked tiles are often what make people reconsider the whole roof, so read about cracked roof tiles before deciding a bigger change is needed. Whatever we fit carries a 10-year workmanship guarantee, in writing.