Does your local zoning and permitting office allow conversion from a single family home into a multi family home? Have you discussed the fire breaks and sprinkler system with your contractor? Or were you planning on doing this illegally and unsafely?

Egress ?

My sister had a house with a basement apartment - it worked fairly well (and legally) because it was a walk out basement, with plenty of light and easy ingress/egress that didn't involve going through the house upstairs. Her MIL lived in it for several years.

Sophie: This forum is going sideways!

Will it be legal and safe? Check with your code department about requirements. Will it make financial sense? Fixing a damp basement costs big money. Life will be much easier if there's a separate heating/cooling system and separate water/electric/gas meters and they cost. Sound-proofing, or the lack of it, can make or break a basement apartment, so budget for that.

10x20? Look at travel trailer floor plans.