Check with your local building department.

Most jurisdictions require a garage (or carport) for on-site parking, so conversion may not be legal.

The garage floor may be damp in the winter, causing mold.

The garage roof is probably not insulated, like a bedroom should be.

Building codes require heat, smoke detectors, escape windows, and ventilation in bedrooms.

You will need to plaster the walls as well if they are just grey blocks, and make sure there is proper heating too. I would say a cost of around £1000-£2000 ( $2000-$4000)