Some roof spaces are more easier to convert then others in terms of size, shape and construction.
The major thing you need to have when wanting to convert your loft is head hight. if theres lots of height there’s much more of a certainty that you will be able to convert your loft. The higher the better and even when you cant get planning permission for lets say a rear dormer if theres a good head hight, you can still just convert inside space of the loft and still make a very good sized loft room and maybe even a shower room or bathroom.
The minimum head height you will need to do a loft conversion is 2.2m (7′2″) but you will need a dormer as the space is so restricted.
If your loft is under 2.2m then its pretty much a non starter. It will still be worth getting a quote through from your local builder though as there is a special way not losing any height in the loft called triperling the joists, or you could lower the ceilings but that will depend on the windows on the 1st floor and if theres enough space.
You achieve this dimension by measuring the top of the ceiling joists to the underside of your ridge board.
Some builders you might come across may say they can convert your loft even if it a bit smaller but please do be very careful. If you decide to take your builder up on his offer do make sure you get some sort or agreement between you both before the build commences the works.
Imagine being 3 or 4 weeks into the build and the floor and stairs have finally go in… the last thing you want to be arguing about with your builder is the new bedroom head height. He’s promised you a good head height but it ends up being 5′6″
The reason you need 2.2m or more is because once the new steels and the floor joists have gone in over the existing ceilings, the head height will easily be reduced by 150mm (approx).
Secondly you have to allow for all the new plasterboard, plaster, insulation and carpet/wood flooring etc. Taking all this into account, the total height lost in the loft will be around 180 to 200 mm.
Obversely on a 2.2m high loft a finished head height would be around 2m (6′6″) which is fine if your only 5′ tall but if someone was 6′4″ was buying your house it may be a problem for the purchaser if they cant use the loft room properly…
There are a few ways to convert your loft without having a good head height though but please do take professional advice and discuss all of your options before beginning the loft project.