First fix happens when the structure is up and the walls are open. Supply and waste go to the exact positions of fittings that in many cases have not been bought yet, so the positions are committed from drawings. Floor voids get their runs before the floor closes. A slab gets anything passing through it before it is poured, because afterwards it is a diamond saw.
What has to be decided before first fix, not after: where every fitting sits to the centimetre, which way the WC waste runs, whether there is underfloor heating in the build up, whether the loft tanks are staying, and where the boiler and any cylinder are going. Changing those later is the difference between an afternoon and a fortnight.
Building Services is priced after the job has been seen, anywhere in the areas we cover. Send the postcode and a few lines about what it is doing when you contact us, and the answer comes back in writing.