My surveyor has put a retention on the property value
This is a way of saying he or she believes the property is worth £X but would not recommend the bank lends all of this until a particular problem, say the damp, is checked.
The amount he is suggesting gets held back is known as the "retention" but it is not meant to suggest that this is how much any suspected works will cost - it is a usually a much larger amount to cover all eventualities.
So lets take an example where you have offered £200,000 for a house. The surveyor has said he agrees the property is worth £200,000 but he has placed a £40,000 retention on the valuation until the roof is checked out by a specialist.
£40,000 is, in this situation, probably double the cost of a new roof but as the surveyor is not a roofer he can't be accurate and so needs to go high on the retention - a kind of "just in case and then some".
You'll need to organize for a roofer to go round and give their take. They might say (and they often do) everything is fine. They might punt for a bit of work because ... well its a roofing company.
You then send the roofers report to the surveyor and he removes the retention.
On the other hand if the roofer says the current roof has come to the end of its natural life and a new one is needed for £20,000 you can go to the vendor and say "I didn't know about this" and you can see of there is space to renegotiate.
At the same time you forward the report to the surveyor who changes the retention to the real figure of £20,000.
All being well you renegotiate the buying price to £180,000 or somewhere close (remember roofs generally need major work every 20 years so the vendor might not be willing to help you fund all the work!) and you have a survey valuation that will allow you to buy the property.
To find out more about how to handle retentions pick up a copy of my ebook How to Really Buy a Property.For more information read the section on retentions in What A Property Survey Really Means.