Garage built on side of house >> habitable room ?

I suspect so yes, but it shouldn't be a huge problem.

Tae some photos of it and phone up the planning lot and send em pikkies if they feel its in their remit.

The house has previously been extended a couple

Stoothing?

Better stud it/plasterboard it and insulate between - far more handy, probably warmer and room for cables and pipes..

drains. Make sure you think drains..please no manholes in the floor..

The main planning obstacle is likely to be parking. You're depriving the house of a garage (1 parking space) and increasing the bedrooms.

Owain

Seeing a car parked in a garage these days is quite rare:-)

They are the people who said that housing built on the station car park (St. Albans) would not need parking provision because they would commute by train.

regards

The floor. Is there space enough (below the house floor level) to add insulation then concrete? If so and the floor's easily removed then it might be an idea to do this, if OTOH you've only got 150mm (the norm) and the floors already concreted, then it's probably not worth it.

Also I wouldn't be arsed building another leaf, just dpm the wall add studding and 100mm kingspan, plasterboard and skim

You may have considered this ...but you will have to apply for 'Change of use'

This will be the planning dept ... , this is seperate to Building Regs

'Ee's from Yorkshire, they talk funny.

Insulation.

-If you're building a new inner leaf, it's unlikely to have the 100mm cavity that would be required for today's level of insulation. You might be better going with dry-lining.

-Likewise the floor. If the difference in levels is 100mm, you would struggle to get insulation + screed, so you would have to go for insulation between battens. Is there a damp-proof membrane in the existing slab?

Unless the new room is accessed off a hallway, the window would need to have an opening wide & tall enough for a means of escape.

Smoke detectors will be required in the existing building.