We had our attic converted 4 years ago – 2 double bedrooms and a wc. House was built in 2000.
We are lucky that the roof has a high pitch as this enabled us to maximise the headroom in the new bedrooms.
We had the ‘wrong’ rafters (i.e. skinny, cheap ones) but this was overcome by adding extra rafters when the cross braces and ‘W’ struts were removed.
The whole conversion is built on 2 huge steel girders that were installed and span the entire width of the house.
We sacrificed an existing single bedroom to put in a half turn stair and create a study area.
Planning permission wasn’t required as we were fitting Velux windows rather than dormers.
Can’t remember the actual conversion cost but the whole lot including decorating, carpets, fittings, beds and furniture came to about £23k.