What I would do:

1) I would not permit destruction of the existing plumbing except in the
context of a guarantee that it would be replaced by a specified date
with a code approved installation. I would protect myself against default
by the plumber, by requiring a letter of credit to satisfy potential claims
against the plumber if he destroyed the plumbing without replacing it.

2) I would ascertain the average ratio between assessed value and recent
purchase price. I would put 3 Red Barn Road on the market:
offering price = (assessed value)x(average ratio)+ 1.33x(plumbing contract)
and see what happens.