The clue is ce qui (literally: that which), the subject of the clause, which is singular. It refers back to a general idea and is always singular.

J'en ai vu une autre hier qui avait un grenier et un sous-sol, ce qui serait parfait -- I saw another one yesterday that had an attic and a basement, which would be perfect.

Hi Adrienne,

I see "une autre" as being the subject of the following clause and therefore "serait"

"The other was perfect"    because of the enhanced storage facilities.

Bonne journée

Jim