In Europe we've used regulations. There's standardised ways to measure area of an apartment/house, and it has to be included in any ads and contract of sale/rent. If the actual area doesn't match what's in the ads/contract the landlord/seller will be in trouble. I saw a news article once where the landlord,iirc, had to return all rent money to the tenant. The tenant had lived there for a while(memory is fuzzy, I think it was years, but don't quote me on that) and in the end only paid for electricity.

I hear you guys like suing eachother in the states, seems like this kind of regulation would suit you just fine!

Require advertisements for property to include the floor (and ground) area.