I'm sorry to say, and hoping I am wrong Roy but I recall you had a dispute with your builder which you wrote about on CBF last year. If this was not concluded to the satisfaction of all parties (well the builder mainly) it is not uncommon for one party to "tip off" the taxman about something or other. Alternatively it could be this dispute at the core of it and mercury could be right - maybe one of your neighbours has suggested the tax office look into your builder's finances.