If a villager house spawns at x=0 y=63 z=0 and the game generates a 2 meter tall hill at x=-1 y=65 z=1, the game will generate dirt or grass blocks within the village.

When I loaded up a world and found a village, grass dirt was inside a house. It (obviously) only occurs on non-flat worlds. It may have already loaded in an earlier version, so it may be fixed.

Okay, now I made a new world in hardcore in 14w27b. I went to the plains biome to try to find iron. I found this village and it had tons of dirt and grass in it.

Seed = -5925286564204335691
x= 110, y=65, z=592
To fix, include air blocks in the villager house structure code.
A hopefully helpful diagram (I'm a visual learner ):

Where the house spawns:
XXXXXXX
ZZZZZZZ
ZYYYYYZ
ZYYYYYZ
ZYYYYYZ
ZYYYYYZ
ZZZZZZZ

without house:
TTTTTTT
TTTTTTT
TVVVVVT
TVVVVVT
TVVVVVT
TVVVVVT
TTTTTTT
T= no hill
V= where the hill spawns
X= gravel
Y= house blocks
Z= where the game checks to see what height to spawn the house on