Op is my exact situation. GF and I just sold condos and bought a house that will most likely be our retirement house. 8 bike stable between us. Co-habitation, domestic partnership, whatever you want to call it. Both single, no kids, never married.

House is smallish. My choice was a fairly large garden shed, a very large two car garage with portico, or an old Sears box truck I asked the seller to leave. It's just the box, no wheels or cab.

My other hobby is fishing, salt and fresh, and I have a ton of gear related to it. My main man cave for that will be the garage which also has both a gas heat and wood burning stove. (House and structures are 1895 vintage)

Garden shed will be for garden stuff.

Bikes...to the Sears box truck! It's wired with interior fluorescent lights, roll up door secures with a lock. I don't do my own maintenance, so all I need is somewhere secure and dry to hang the bikes. Box truck...Bike truck.