They are a nightmare, when ever I'm asked, i just say sorry i don't do builders cleans, end off.
Most including the op will now say that I'm thinking

Not only that if you take on builders cleans then you are leaving yourself wide open if you don't get a scratch glass waiver signed for each job beforehand too.
Take this example claim for instance

Example 1
Type of claim: Damage to Property Worked On (Window Cleaner)
Outcome: Claim Paid
A window cleaning customer of GLEAMING INSURANCE contacted us to advise that he had recently completed a builders clean of a new commercial development and that his customer had now alleged that 62 of the window were showing evidence of minor scratches. This damage was holding up the opening of the new development and urgent attention was needed.
Our client was worried and didn’t know what to do, he was concerned at the possible cost of rectifying the problem and he was also concerned that his customer would hold back monies due to him for other works carried out.
A normal liability policy from many other insurers would have an endorsement with an exclusion for “damage to property worked upon” meaning that scratching or breaking glass would not be covered, but this is core cover on our policy and something that sets our cover apart, so we were firstly able to reassure him that this kind of damage was covered with GLEAMING INSURANCE.
We then immediately involved our insurers and their liability claims team and we were able to get a specialist repairer and loss adjuster to speak with the customer and his client and arrange prompt repairs to rectify matters. The insurers were able to ascertain that the scratches were not deep enough to warrant replacement, but that they could be repaired and polished out to exactly the same level as a brand new pane of glass – most other insurers would have simply paid for a replacement.
Our client not only kept his customer happy, but received his monies on time and more importantly for the future didn’t have an over-inflated claim on his record for years to come.