single purpose building is a design anti-pattern that manifests as building used for producing one unit or one upgrade.
Generally the opposite of "versatile building" which has both upgrades and units.
Ideas to expand roles of buildings:
1.Add a bonus for controlling the building:extra energy/resources, supplies, unit count.
2.Add a turret or offensive capability(tower). Possible as upgrade.
3.Add more upgrades or units produced in building.
4.Make a building give some defensive bonus: e.g. repair nearby units.
5.Allow units to garrison inside and return fire.