Building Regulations relate to the space and how it is accessed and if you place any sanitary ware they want to ensure the installed works are safe and compliant to meet the requirements of the Building Regulations, they are not interested in the room.
In addition you may need to apply for planning permission if you place Velux windows in the roof slope facing the street.
In respect of the above you will probably have to submit drawings of the proposed works in order to obtain Building Regulation approval.
If you require assistance with this we do offer a design service to ensure BR is obtained.
