You say it's near the kitchen water line. How close is it to the main *waste* line -- i.e. the huge pipe that all the toilets drain into? That's where your big costs come from...

Actually, not a lot further! About 12' total. The only bathroom is just past kitchen. And we're on septic, right behind it all. Thank you!

Tying new into old, and the roofing and siding match is where some big expenses are, besides the obvious plumbing difficulties. Add 20K to your high estimate. 40K in a high COL area.

More than you think. If you have the space, it might be more cost effective to add a bedroom and bathroom. Knocking it out for 5 x 8 isn't gonna be worth it. Adding a bedroom and additional sq ft plus a full bath will add a lot of value. How does septic work - is it sized to handle a certain amount or what? Is there a problem with adding toilets?