A worst case scenario is is was badly built with rubbish in the floor
then a thein ish layer of concrete so you have to replace the floor.
However, sounds like you should be ok. You'll need a brick partition
but you can do this. Use conrete blocks, or if you have plenty of
money, use lightweight blocks. You can get insulated light weight
blocks which together with a layer of celotec and then plasterboard
shoulb meet the required U value. Light weigthblocks are easier to use.
Wall ties every two courses and 2 blocks wide. Do same for garage
opening and put in a window. All DIY stuff except perhaps the outer
leaf of bricks in the garage opening. If this is rendered on the
outside then you can use blocks for the outer leaf also. Extedning rads
piping is easy with pushfit and plastic piping (new rads will require
thermostats).
Applying for planning permission and building permission yourself on
this type of job is relatively easy. Make an appointment to see a
planning officer and get in early (my Building Control Officers are
avaialble 8:30 -10, then they are out doing inspections) to see the
BCO, pick up the forms and apply etc. then DIY or get ina
bricky/builder.