Negotiating the price

Folks who have negotiated the price down - when did you do this? I just put the survey through CoPilot and it came up with some extra context to be worried about - namely the roof needing to be lined. We've just finished enquiries and were thinking of moving to a completion date, but now I kinda want to talk price with them. Is it too late? How does negotiation work? Say 20k off they say 10 you shake on 15? Id love to hear some experiences. Thanks!!