[dragonwriter]

In many jurisdictions there are habitability requirements that, if not met, make it unlawful for the landlord to collect rent for the period during which they are not remedied, but there I am not aware of any that requires the landlord to supply substitute lodging (the presumption generally being that the withheld rent can instead be used for that purpose, though that’s dubious, in general.)

[bdowling]

It’s a simple breach of contract. A rental agreement basically states that the tenant will pay rent and the landlord will provide a habitable apartment. The most common breach occurs when the tenant doesn’t pay their rent. However, when the apartment isn’t habitable, it’s the landlord who is in breach. The damages in such a case are the cost of substitue comparable accommodations, even if that cost is higher than the rental amount. So the landlord may not have to provide substitute accommodations, but he would be liable for damages if he didn’t.

Edit: I may be biased by living in California where there is apparently an implied warranty of habitability.