So far the easiest work-around we have for this is to give your faction a static resource that is only required by that particular building or unit. For example, let's say your faction has a starting resource of 1 Soap, and its Bath House building requires 1 Soap. When that building is built, the faction's Soap value goes to 0, and when the building gets destroyed it will go back to 1, so another Bath House can be built. Static resources work like library books, basically. When you build a unit that requires that resource, it "checks it out", and then when it dies it "checks it in", and the resource becomes available again.