Second time around, it didn't work but I did find another way of doing it, it's a bit longer to do but it worked.
I had to kick my family out of the house, save the house and bulldoze the lot they were living on, as the tombstones didn't stay on the empty lot under freebuild. Then stick the house back on the lot and move the family back in.
It's a bit of a pain to do but it works when you can't delete tombstones and both what you Playalot, and MrsFlynn suggested doesn't work.