We didn't go with one firm for everything, but took good recommendations for the trades we have used so far, and have been more than happy with the quality of the work from all of them. They all know their stuff, but perhaps our naivety of the building trade and the poor descriptions on quotes apparently leaves large gaps between what each trade has signed up for, which all can be sorted out for a price... The architect/structural engineer has been overseeing the project, but obviously his time costs too.
We haven't changed anything from the original plans ourselves.
A couple of examples.
Day three of the ground workers on site, one of them says. "what about the oversight? The boss doesn't usually include that in his quotes." I had no idea what 'oversight' was so had to google it. Apparently, the foundations concrete stops a couple of feet below ground level, 'oversight' is the name given to the few tonnes of earth that's higher than the concrete level within the boundary of the building. They can remove it (within half a day as it turns out) for the sum of about 2.6k. That and the fact that certain parts of the foundations needed to be deeper than the plans said due to tree roots, meant the ground works was a few thousand over budget. They then tried to fleece us 4.5k more for the surface drainage (rain water) that was explicitly excluded from the original quote. £5k to dig a hole and fill with peas shingle? (we declined, I'll dig it by hand for 5k)
Another example.
Just last week, we've paid for the soffit and fascias. We had already committed to the windows colour, and wanted these to match. Who'd have thought that a dozen lengths of plastic could cost over 2 grand? I priced the guttering for the whole house at about £100 from Wickes, for a similar amount of plastic. But again, with the roofers saying they are being held up by the chippy, who is waiting to fit the soffit and fascias just as soon as they turn up, we feel like we're paying ransom prices again to keep things moving.
The list goes on.