So I'm Getting 1600 Square Feet... Way too long

I’m loving the conversation.

If you can get a bowling alley (2 lanes?) into a 40x50 footprint, you’re hired.

We can compromise, and we can be neighbors. I could make the kitchen the exercise room, and the bedroom the workshop. That would preserve the investment I’ve made in the kitchen.

In case I wasn’t clear, there are 4 main and separate places down there:
Kitchen, Bedroom, Bar (which is 15x25 and serves as the living room for the apt), and bathroom. The rest of the space is taken up by things like the boiler room, landing at the bottom of the stairs, back hall to the exterior door, etc

It is. And the open floor space in the kitchen is about 15x20; the bedroom is a bit of an odd shape, but probably “averages” 15x15

It’s not legal, and as it is there’s no safe way to separate this space from my space. So it’s family only.

You are correct. There is a long story behind us buying this house. It’s massive, and it’s in a very working-class area. We bought it for a variety of reasons including location and its ability to accommodate the entertaining we do. We had to buy what was available, and will downsize for retirement. But I’m not dealing with moving more than one more time. Next one is last one. It’s 5000 sq ft (incl the illegal apt) and when the time comes we’ll go back to about 1500.