Two granite walls? I have yet to see a wireless signal that can do that; with current standards, it would almost defy physics. After all, it is only a Ghz-range radio transmission. What you really need to do is run Ethernet cables between the buildings, enclosed in flexible pipes of some kind to keep them safe and allow easy replacement. Then a stick wireless access point at the end of the cable in each building that needs wireless access, or an Ethernet switch/hub if you decide that wifi is not needed. Since you are renovating the site, this seems like an ideal opportunity to install a really reliable solution.