Mostly a case-by-case thing, I think. I know (from watching Time Team) that in Britain there are a fair number of fields which the farmer can farm, but can't di

Mostly a case-by-case thing, I think. I know (from watching Time Team) that in Britain there are a fair number of fields which the farmer can farm, but can't dig deeper than that due to the archaeology. Other cases have existing buildings ("Oh, by the way, you have an iron-age cemetery under your house. Enjoy!"). In cases of new development, it would be investigated pretty thoroughly to determine whether it should be protected (involving negotiations with the developer), or either entirely excavated or just left and the development continues.