Converted garages that feel cold, smell musty or show damp patches at floor level are usually suffering from a specification shortcut rather than a lack of radiators. The causes are well understood and mostly fixable.
No damp-proof membrane in the floor
Garage slabs are often laid without a membrane tied into the wall DPC. Boarding straight over that slab traps moisture underneath and it shows up at skirting level.
Single-skin walls left unlined
One-brick walls have no cavity and no insulation. Without an insulated inner leaf and vapour control they run cold and attract condensation.
Cold bridging at the front infill
The panel where the garage door was is a frequent weak point, especially where insulation is not carried continuously through the new construction.
Ventilation
Rooms converted from garages often lose the background ventilation the garage had, without gaining trickle vents or extract in its place.