Two separate approvals sit over a loft conversion and they are constantly confused. Planning is about whether you may change the appearance and the use. Building control is about whether the thing you build is safe and warm. A dormer can avoid the first entirely and never avoids the second, which is the single most useful thing to understand before getting quotes.
What permitted development can cover
Householder permitted development rights allow a certain volume of additional roof space on a house, subject to limits, and a rear dormer within those limits can be built without a planning application. The limits are specific: a volume allowance that differs between a terrace and a detached or semi detached house, a rule against extending beyond the plane of the existing roof slope on the principal elevation, a requirement not to go above the highest part of the existing roof, and set backs from the eaves. The technical guidance is the document that actually decides it.
Where the rights do not apply at all
Permitted development rights for roof extensions do not apply to flats or maisonettes, and they are restricted or removed in conservation areas, national parks, areas of outstanding natural beauty and on listed buildings. They can also be removed street by street through an article 4 direction, which is common in areas where the authority wants to protect a consistent roofline. Your neighbour's dormer is not evidence that yours is permitted.
Full width dormer permitted development and the volume allowance
A full width rear dormer is frequently built under permitted development and frequently is not permitted, because it is the option most likely to exceed the volume allowance or to breach the set back and eaves rules. It is also the option most likely to be noticed. Where the design is close to the limit, a lawful development certificate is worth the fee: it is the local authority confirming in writing that no permission was needed, and a buyer's solicitor will ask for it.
Building control applies either way
Even a conversion that needs no planning permission needs building regulations approval, because it creates a habitable storey. Structure, fire escape, the staircase, insulation and ventilation are all assessed. The completion certificate is the document that matters when you sell, and a loft room built without one is routinely described by surveyors as a room in the roof rather than a bedroom.
Questions people ask about dormer loft conversion planning permission
Do you need planning permission for a dormer at the rear?
Often not on a house, if the design stays inside the permitted development limits and the rights have not been removed where you live. On a flat, a listed building or in a conservation area, assume you do and check with the local authority.
What is a lawful development certificate and should I get one?
It is the council confirming that your proposal does not need planning permission. It is not compulsory, and it is cheap insurance where the design is near the limits, because it is what a purchaser's solicitor will want to see.
Does a loft conversion without dormer avoid planning?
A rooflight conversion that stays within the existing roof plane is much less likely to need permission, which is one of its attractions. You give up the head height and the vertical wall that a dormer provides.
Does a neighbour's dormer mean mine is allowed?
No. Theirs may have had permission, may predate a restriction, or may be unauthorised. Permitted development is assessed against your own house and its own planning history.