The main by-products of the coking plant are only benzene series and coal tar, and the brothers are smoking every day. Even if there is a legacy, it is only contained in the soil layer. When building a house, some topsoil layers have been removed together with construction waste. In addition, water is probably the most influential by-product, but don't drill wells by yourself, use piped tap water. How big can the problem be?
When buying a house, deliberately hype this topic, but there is an opportunity for developers to reduce prices.