This particular model (in this state 15k polygons) can be optimized a lot more without quality loss, there is a lot of unnecessary edges, loops, and a lot of unnecessary modeled details that can't be seen from a distance at which usually this type of model would be looked at, and those details should be done using normal map If we are talking just about optimization, just filling the safe frame of a render with it will cause rendering of unnecessary polygons because even at that distance you can't see all of the details. If this is meant for archviz exterior, it can be used as it is now, but if this is meant to be used as a multipurpose asset (real time, games, archviz etc.) 15k is a bit high for what it is. (Answer based on google translate translation)