Answers There is a general requirement that the product be fit for purpose. Tanalith treatment is not a requirement but would be considered normal good practice. It would depend on what was the essence of the contract as to whether or not this could be enforced in court - unlikely in the extreme but just possible if the contract was, say, "please build me a high quality extension in accordance with the best practice" . If, however, it was "Please build me an extension as cheaply as you can that will not fall down but be ok" then there would be no obligation to use tanalised. As a matter of simple competence, given the nominal price difference and the huge increase in quality, any builder that did not automatically use tanlasied is demonstrating his lack of competence. BCOs are generally almost as ignorant in matters of preservation as builders and should not be relied on. When they got rid of the proper old DSs in the mid 90s they put in young lads who were clueless and paid them peanuts. Wall plate on a dpc has been in building regs since about 1886!!!
Answer to your last question >Why would any builder spend extra time and money on
a good builder would do so as a matter of competence and protection of his reputation a bad one will do a cheapskate job
I refuse to work for clients that want a botch job done I can afford to do so because clients appreciate it done right and luckily I have plenty of them
chris