Typed from the middle of a building site...
Our plans were drawn up three years ago by the architect. It then took three years and countless visits from builders, 80% of whom never even got back to us, and the other 20% were split between "site is too difficult*, so no ta" and "site is too difficult so here is 25% - 150% ott estimate" before we found a builder prepared to do it. (and then it is a risk - he is young and this is his first major project after leaving his Dad's building company to strike out on his own).
Building Regs have all manner of tricks up their sleeves - our central heating boiler has had to be moved to the loft**, ffs, to comply with the rules, and then there are the usual surprises when you find a floor joist is rotten (leaking shower), a lintel is missing bodger builder from before us, or the Southern Water drains are 10m closer to the house than shown on the Southern Water plans (Meaning wait 12 weeks and get SW to move the drains at an extra cost of several thousand pounds or build the extension .5m shorter than planned. Ssssh! Don't tell the architect, we are going with the latter.)
tlh and I have a good working understanding; any key decisions on the build, e.g. replace that joist or repair it (building control says 'up to you', I get the casting vote, any key decisions on the fit out, she gets the casting vote (tbh I could care less, I've already picked the range that is going in the relocated kitchen which is all I care about), otherwise she is queen of design, and I am king of finances/paying for it. One couple we dearly love fell out with their architect who was pm'ing their build, great architect useless spineless PM, then when they tookover PMing it themselves, they fell out with th ebuilder - I think they were wrong to try to go for a fixed price build - and once it was done, she loved it and he had fallen out of love with the house and wanted to sell it and move on. They split up over it. She bought him out. Tragic.
Luckily I work away from home so am spared living on-site much of the time. We have four working adults round ours, the joys of the boomerang generation, so it is tough on the other three, and tlh comes to cph as often as possible so as to avoid the noise and the dust as much as possible.
...right. I'm about to go and use the new shower that was installed last week
*three storey terraced house, rear extension, only rear access is a narrow alleyway between us and next door, no on-street parking, only just, and I mean just, enough room out front for a skip, loads of demo work to do both internal and external, et cetera, et cetera.
**but wowsers it makes for a powerful shower now the boiler is above the bathroom!