Jan wrote:

Our house is on the corner lot and around our fence/lawn is the drive
way in the back and on the side. We have noticed that the cars while
going on the driveway are invariably trampling on our lawn. We have
notified our builder and he said he couldn't do anything as it is not
under warranty (we just moved into our new home 2 months ago) but
suggested that he could put a boulder on the lawn for $75.

He's gouging you. You don't need a huge boulder (nor would you want the
liability), and they're free for the taking everywhere.

Is this kind of problem not really under warranty? Is his suggestion
good to follow? Otherwise what can we do ourselves (wise and cheaper)
about it?

The boulder worked for me.

Also consider planting a whippy shrub, like a Forsythia. Won't damage
passing cars. Likewise, any damage to the plant will be minor and
will heal rapidly.