Steve,

There are a number of considerations to take into account when undertaking a
job like this:

1. It will need building reg approval
2. If you are considering a dormer window it may well need
planning permission
3. you will need to get a professional to draw what you want complete
with
any load bearing calculations
4. You will need to have some good, multi-trade skills if you are doing
this yourself.

After seeing a number of "botched" conversions and one or two of these with
ceilings on the point of collapse, I would advise at the very least to hire
the services of a good professional to advise you on the design and load
bearing parts of the project as floor joists need to be inserted, ceiling
traps and stairways installed, possible re-siting of water tanks, electrical
wiring and, often overlooked - the safety of any chimneys or gas flues that
will project into the converted area.

And lastly they will be able to steer you through and building control
applications/problems - as a SAFE loft conversion is not the simplest of
projects for the average DIY'er to undertake

There may well be some disagreement with me on that point :-)

As regards to costs - certainly four figures and possibly five figures
dependant on the work required.

Hope this helps

Brian

You'll also need to consider fire regulations (perhaps this comes
under building regs). I think you need fire doors on sprung closers
if you're adding an extra habitable floor. Someone may well correct
me, that's fine. Just concerned you don't miss this.

I know I thought about this at our previous house and quickly went off
the idea due to all the factors already mentioned in this thread -
definitely not a trivial operation!