The short answer
Altering the shape of a roof to form a mansard usually needs a planning decision, and in Bath the conservation area, the World Heritage Site setting and any listing make that decision stricter. Repairing an existing mansard like for like is a different matter. Because the rules turn on your address, check with B&NES planning before any design work begins or money is committed.
What counts as a mansard, and why the question comes up
Picture a roof that bends. The lower slope on each side is steep, sometimes close to vertical, and the upper slope sits back at a gentler angle. That bend buys a full-height room under the roof, and it is why owners with a plain pitched roof start to wonder about converting. Our guide to mansard roofs covers the construction, including the flat section hidden at the top.
Planning questions arise in two quite different situations. One is a new mansard, where a simple pitched roof is rebuilt into the two-slope shape. The other is an existing mansard that needs work: a new covering, new dormers, an altered parapet line. Officers treat these differently. A new mansard changes the outline of the building, and outline is what planning policy reads first. A repair that keeps the outline sits in another category altogether.
Planning permission and Building Regulations are separate
Owners often talk about "getting approval" as if it were a single hurdle. It is two, run by the same council but asking different things.
Planning looks at appearance, height, neighbours and the character of the street. Building Regulations look at whether the finished work is safe and efficient. A mansard conversion meets both. The new structure has to carry its loads, the new room needs insulation to current standards, and a new loft room above two floors also raises the escape question, and a roof window serves as a way out only close to the ground, about 4.5 m at most.
Consent under one system never stands in for the other. For the Building Regulations side you can use B&NES building control or a registered building control approver. We explain which parts apply and handle that paperwork when the work is notifiable. Planning itself belongs to the council, so ask B&NES directly and file the reply.
Bath's conservation area and World Heritage setting
All of Bath sits inside a UNESCO World Heritage Site, yet the city-wide conservation area takes in only about half of it. Some addresses are inside and some are outside, and nobody should guess which. The B&NES planning map settles it in a minute.
Within the conservation area, officers weigh what a roof change does to the view from the street and from neighbouring buildings. Lambridge Place shows why. On that terrace the Bath Preservation Trust remarked that built-in dormers on a mansard stand out from the street more than a pitched roof recessed behind its parapet. What stands out gets examined.
Some streets also carry Article 4 directions, which withdraw particular permitted development rights to protect special areas. Examples on the council page run from porches to sheds, via windows, doors and paint. No roof item appears, though that proves nothing about your own street, so use the council's Article 4 page and policy map to check.
If the house is listed
Listed building consent is its own consent, separate from planning permission. B&NES says the two are not the same thing, and it asks owners to phone the planning team before touching anything. Skipping consent can bring criminal liability, which makes this a question for week one.
For a listed mansard the reviewers care about the covering as well as the form. The Trust welcomed natural Welsh slate replacing artificial slate on the Lambridge Place terrace, and welcomed fresh lead in the valleys of a Grade I house in St James's Square. Both of those are single casework comments and not policy, yet they show a consistent instinct: the original material, put back well.
It is the same instinct we work to. Slate stays slate and lead stays lead. For a listed roof we lay out the survey findings first, then the likely council expectations, and only then talk figures.
Repairing an existing mansard
An existing mansard that only needs repair is in a different position. Under the Planning Portal guidance, re-covering or repair does not normally need Building Regulations approval while less than 25 per cent of the building envelope and less than 50 per cent of the roof are affected. A covering that adds 15 per cent or more in weight needs approval plus a structural look, so a heavier material is a decision to weigh up rather than a quick swap.
Whether a repair needs a planning decision turns on appearance. Matching slate for slate keeps the look. Changing the covering, reshaping dormer cheeks or raising a parapet may not. On a listed house, ask first and do not assume.
Leaks are the usual reason for work on an older mansard, and the culprit is often the flat section hidden at the top. Our page on why mansard roofs leak shows where to look.
What to ask before you commit
Go to the council before you go to a builder. Ask B&NES planning three things: is the address in the conservation area, does an Article 4 direction apply, and does your proposal need planning permission, listed building consent or both. Request the reply in writing.
Then have the existing roof surveyed. Hidden faults shape both the design and the cost, and a conversion can uncover rot in old timbers or a flat section that has to be rebuilt. We survey free, send photographs of your own roof and come back with a fixed price on paper, so drawings and budget both start from the roof as it truly is.
If the roof is also dipping, that bears on whether it can take a conversion at all. Our sagging roof page explains the usual causes, and the wider pitched roofing service covers tiled and slate roofs from survey to finish.
