So when I bought my bog standard house with attached garage, the garage was not part of the property? It was built at the same time as the house. The front part of said garage had a garage door. The other end had a shower toilet with a small utility type area and door to garden, access into there from kitchen. You could not live in the garage as it was single block, no plastering, no heating etc. How was the garage not considered part of the house? It had a flat roof.I mean not habitable means it's not considered part of the original house (just as you're not allowed include non-habitable space in floor area calculations) and so falls outside the criteria for rent a room relief, where as if you section off some of the habitable space into a self-contained unit, you can then opt out of the requirements to register under the Residential Tenancies Act.
It's complicated by that fact that planning permission is required to create a self-contained unit, and such planning where granted usually forbids the letting of that unit.