You bet ,the Rad Alt will be affected be buildings as the radiated signal from the Rad Alt antennas will bounce off the roofs just as well as the earth .
Some Autopilots only use the rad alt for the last couple of hundered feet as a hight above runway for auto level out.
The actual glide path used by the autopilot is sensed via the Airports ILS system that coprises of a localiser (left/right deviation from runway center line) and glide slope . These are fed In to the Autopilot along with other Input to allow the Aircraft to fly a predetermind path to the end of the runway.
As the pilot flew over a building the Rad Alt Indicator would show his hight above the roof but would not effect the Autopilot .Ground Prox Warning may be affected ,Iwould have to read up on that one.
Hope this helps.