Any Civil Engineers here?

[mikaztro]

Wall to wall or "firewall" consrtuction is legal as long as you follow certain minimum standards like 0.5m height of firewall above from the top of the existing roof. Moreover, there is a minimum use of firewall, only two sides of the property. So long as you follow the building code requirements and city ordinances, there is really no problem at all.

Flakes: You better check the construction of your common wall. It should not have any cracks at all even if your neighbor's wall is sagging or drifting downwards unless your house construction is sub standard which I doubt. Usually, both party walls should have individual wall foundations so as not to impede the structural integrity of the property walls.