Airports which are certified for autoland usually have a reasonable amount of flat ground before the runway threshold.
Baro is not used for autoland because it is not accurate/consistent enough. Rad Alt-based navigation doesn't rely on the pilot dialling in the correct barometric setting for the airport (QNH) every few minutes.
As for buildings, I'm sure the autopilot system is sophisticated enough to ignore momentary height variations. I know it will ignore the loss of ILS signals for a certain time period- it assumes that something (e.g. another aircraft) has briefly blocked the signals. There are a lot of intelligent algorithms like this built in to autopilots these days.
Rgds.
NSEU