hi there
all trades on developments end up signing the developer contract, on there they will stipulate the retention that they set out.
i agree with you, we used to argue that the retention on the clean was pointless, as the propoerty is only clean for about a day after it as been completed.
However, when you are cleaning sites worth of properties for your clients then you agree or they go elsewhere.
the construction industry, and especially housebuilding has always been like this, when it all went into melt down a few years ago, developers immediately cut costs, asking trades to retender for the balance of the sites, at a prescribed lower level, others were asked to accept a discount on all outstanding accounts.
make sure that you are fully belt and braced for the work and if you can make profit from it then great.
regards
Martin