Something like this would be extremely cheap to do , plus they are insulated and can easily be bolted down onto a slab in areas where the ground does not freeze.
The biggest problem is getting permitting for something like this because it is not conventional . I was the contractor on my own house and you would not believe the nonsense they put you through , even out in the country away from everyone .
Had to fight the VA for three different vets in my family. It's atrocious what they put you through.
Used to be that families of multi generations lived together and helped each other out during times like this . Now many places do let you do this even if you own the place , this needs to change. To many busy bodies out there.