N-scale is the rough equivalent of 9 mm. I've used 10 mm buildings with 15 mm without a problem. Of course, if you have the 18 mm version of 15 mm it might be a bit more obviously out of scale. However, smaller footprint buildings work on the gaming table since they are out of scale for everything except one to one skirmish anyway. I'd say go for it as long as the price is right.