A factory building is needed. The factory makes kits for others. For example, a kit for spell factory. You attach the kit to spell factory then making time of a spell reduces.
Or a kit for mortars to cover a wider range.
or a kit for any building to increase hit points of buildings
and many others.
Of course there should be limitations. For example, only 8 kits per village are allowed and kits should be made at a very high cost.

