The most common and most likely answer to this crossword clue is the 6 plus 3 letter word VACANT LOT
I've seen this crossword clue in Independent
V
A
C
A
N
T
L
O
T
More clues leading to the result VACANT LOT
Buddy explains!
The straight part
We know that "building land" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case building land = vacant lot.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.