A basement will probably be more because you have to break up concrete to run new plumbing.

I am looking to protect her and myself just in case there should be a breakup.

I like that a lot because it is much easier to handle that during the initial lease signing, when people think they love the place and they will stay through the lease and maybe even longer, before their divorce or break up with their boyfriend... etc.  

If you use an excavator to knock down the walls, the same machine can dig/break up the bottom in about 15 minutes.

Low slanting roofs such as on the dormer are always a problem area for leaks.It might also be possible to put a narrow balcony in front of the dormer to break up the flat appearance.

Again there are many reasons for selling including death, divorce , partnership breakup, balloon due, capital needed for new project, retirement, and on and on.

What happens if they break up and tenants move out leaving boyfriend and kids behind, not paying rent?

He did give me a breakup of all costs and it came in at around 46% for the last 2 years.

Also: if you're going to go into business with your girlfriend be sure you have a good contract to govern the split of assets if/when you break up.

In my first 2 years of renting I have had 3 move outs. 2 were bf/gf breakups that ended end death threats ; abuse ; etc and these are on high end rentals.