If there are any additions or unusual planning aspects to the property, its worth waiting for the contracts to issue before arranging for the survey - the contracts should show details of any planning applications, exemptions, unauthorised developments etc.,etc. which will assist the surveyor.
I'm inclined to advise clients always to wait for contracts to issue anyway before arranging a survey - if there are other issues ( e.g.title, marital, taxation) that may make the purchase impractical, they will save the cost of the survey. That can be awkward though if there is a tight closing date.
mf