Not surprisingly, the financial consequences can be pretty steep. It is not just the cost of remediation you should consider. Many main contractors/developers will be reluctant to take on the risk associated with a site infested with Japanese Knotweed. Those prepared to take the risk inevitably price the risk, which of course gets reflected in the purchase/tender price, and may qualify for some tax relief to turn unusable land into development land.
If the risk remains with the client and Japanese Knotweed is subsequently found, then additional costs during the construction stage are almost inevitable to cover professional fees, considerable management time, additional site precautions and delays to the contract, plus of course the cost of treatment and/or remediation. The cost of remediation in these circumstances escalates due to the urgency, as the more cost-effective alternatives to dig & dump may not be available.
Removing the problem prior to opening the main building contract is the best solution all round.