Leicester has some genuinely good lofts, because the Victorian and Edwardian housing was built with roofs that had proper pitch and proper height.
On the villa stock around Stoneygate and Clarendon Park a conversion is often the best value space in the house, since the roof and walls already exist. Head height still needs measuring rather than assuming, because floor build up and insulation take more out of a roof than people expect, but the odds here are better than on most stock we see.
The constraint is the frontage. A dormer on the roof slope facing the road is not permitted development anywhere, and inside a conservation area, particularly where an Article 4 direction is already controlling external alterations, anything that changes the roofline the street can see will be examined hard. Rear dormers and rooflights are a much easier conversation and are usually where the space wants to go.
The interwar and modern housing at the city edges is trussed, which means the structure has to be reworked rather than simply strengthened. That is routine work, and it is not the same job or the same price as a cut roof.
The permitted development volume allowance is forty cubic metres on a terrace and fifty on a semi or detached house. Escape, fire separation and stair position are all Building Regulations matters. Our loft conversion page goes through the structural options and the order the work happens in.