Not only is there a "homeowners" category in the "Demographics" targeting section, but there are specific sub-categories like "Home value" $50,000-$99,999", "Home type: Condo", etc. IIRC, they get it from a data broker like Experian.
LexisNexis is actually the big one who resells that information, you can request a copy of your report from their site for free to see what they got on you. Mine listed my house, I believe the day I bought it, and the source of that information (in my case my town hall).
How? Their site has a physical form and instructions to mail it - which is so fucking ironic given they freely sell our data in digital form but we need to mail them forms to get a copy ourselves.