A client of ours informed us that their offer had been accepted on a house that required a lot of work. They wanted to find out at what stage should they be looking to employ an architect or a builder to go into the property they wanted to buy as they were keen to get them lined up but obviously they did not want to irritate the vendor at a delicate stage but wanted to know what the work would cost in order to ensure their offer price was the correct one.
Vendors would generally get extremely annoyed when buyers reduce their offer at a late stage because of something they could have taken into account from the outset. The best practice is to have a reasonable idea of the cost of works before agreeing a price. Yet is is not unreasonable to request a visit with an architect and builder at any stage of the process.
Should the cost of the desired work not have any influence on the agreed purchase price then it would be correct to wait until after exchanging contracts before bothering the vendor. However, if you believe you want to revise your offer baased on what the builder and architect says then get them in as soon as possible.