Floating floor versus traditional footings

I have a very old and tired double garage that i'd like to convert to a bedroom/ensuite. Much of the brickwork/mortar has perished, riddled with damp, roof leaks in places and started to crumble so I decided to demolish parts of it and rebuild.

My question is, do I create a floating floor to the existing slab (slab is solid and good condition) and rebuild parts of the single skin, and insulate as per regs, sort the roof out or do I demolish the lot and rebuild with footings/twin skin blockwork and cavity etc...

What is the cost effective route?

TIA

Start over if the old ones that bad: probably keep the slab as you suggest

thanks for the reply, most of the garage needs to be pulled down in all honesty but I'm trying to figure out the best route cost wise. Take out what's there including some footings and replace or just build up off the slab?

Rebuilding like for like will be more than double the cost, for about the same space and amenity.

Rebuilding may attract planning permission requirement.

If the floor slab remains, it will need insulation and a floor deck on top which will decrease the room height if the ceiling is at a fixed level.

Does it 'need to be pulled down', or is that just what you think? If it does need to be pulled down, then the decision is already made, but bear in mind that it does not necessarily need to be replaced with masonry on new foundations.