Basically, when you enter it, Build mode and Buy mode won't lock out during fires, burglaries, and whatever else will lock them out.
After you open the cheat console (Ctrl+Shift+C), you type in "AlwaysAllowBuildBuy On".
If this doesn't work, try "AlwaysAllowBuyBuild On". Like some others, I get confused with buildbuy and buybuild.
I also think you need TestingCheats on to do this.
EDIT:
I think that description was vague.
So, when AlwaysAllow is turned on, Buy Mode and build mode will not lock out when a fire or burglary occurs.
To turn it on:
1. Open the cheat console.
2. Enter "TestingCheatsEnabled True"
3. Press Enter.
4. Open the cheat console.
5. Enter "AlwaysAllowBuildBuy On"
6. Press Enter.
7. If that fails, try "AlwaysAllowBuyBuild On"
Hope this helps catlover.