I'm not sure I will be able to help, but I have a few questions to ask to help me get a better idea of what is going on.
Does this only happen when placing lots? If not, list what you were doing right before the crash has happened
Is it all lots or any lot, maxis or custom?
Does it happen in both downtown and the main neighborhood?
Did you download and install any CC recently?
Do you have any mods (or have you downloaded houses from the exchange?)
I have fixed it! I just had to restart my computer : P
Thanks for replying :-)