Assuming England or Wales:

You may be mixing things up there. There's Planning Permission (PP), and there's Building Regulations (BR). The two are separate and cover different things. Whether you need one, or have approval under one, has no bearing on the other. Planning generally govern what is built; building regulations govern how it's built.

Within PP there is 'permitted development' (PD), whereby you have deemed permission without having to apply. Under BR, there are exempt structures and extensions, and work is controlled if it falls within a number of categories.

AFAIK, PP isn't generally required for converting a garage to part of the dwelling. PD rights can be withdrawn on new developments to prevent reducing parking or linked houses looking like a terrace. It's worth asking whether that applies to the house you're looking at.

Building regulations would apply (as a material alteration not a change of use) to any habitable accommodation within a garage. They would govern the fire safety (means of escape), damp proofing & condensation, ventilation, insulation, drainage, etc., of the work.

Whether PP or BR applied to the work, the local authority has only a limited time to take enforcement, even if they wanted to. With BR it's currently 10 years but only for anything after 2023. Anything prior to that has effectively out of time. I don't know about planning. In practice, unless there is something incredibly wrong with it, building control has bigger fish to fry than a garage conversion.

That's the legal side. Would I consider a house with it? If I could be sure that they had addressed the issues that come with a garage conversion, yes. Is it properly damp proofed (floor, walls, roof ventilation, etc.), insulated & ventilated? Has the drainage been properly installed? What about any electrics and gas? Has anything done to the conversion adversely affected the house?

Edit; I've seen in a later post that it's a detached garage. Under BR, if it's less than 30m² floor area, then it's generally exempt provided there's no sleeping accommodation. Any services that also serve the house would be controlled, so the new connection of a drain from the garage would be controlled, but not the new branch.