If you are considering a loft conversion under Permitted Development rules, there is a limit on the additional volume that you create by forming a new room in the loft space. This limit is 40 cubic metres for terraced houses and 50 cubic metres for detached and semi-detached houses. Any previous roof space additions must be included within this volume allowance. Although you may not have created additional space a previous owner may have done so.
If you want a dormer-type loft conversion that exceeds this limit, you will need to submit a planning application. The Planning office will take into account various factors, including:
- type of house ie terraced, semi, detached
- type of area you live in eg conservation area
- size of house
- size and type of the intended dormer
Typically, the Planning Office is sympathetic towards discrete subordinate roof additions.