[michaelt]

If it's your project, you can order the builders to reconfigure the ventilation ducts pretty much however you like.

Building control require that an architect sign off on things like the size of the beam holding the roof up. If you change that, the architect will have to sign off on the building as constructed before building control will approve it.

Planning permission is given on the basis of external appearance, so you can't change too much in that area - but go look at some planning applications, they're very light on details.

But other than that? If it's your project and you tell the electrician putting in a socket to put it somewhere else, he'll tell you if it's legal under the wiring regulations, and if it is, he'll move it.

The main reason that in large projects builders are required to build exactly to the plan is to stop them substituting cheaper materials. They're under great pressure to do that, because if you allow your builders to replace expensive zinc cladding with cheap ACM cladding, and you ask several builders to tender and choose the cheapest builder, you'll always choose the builder who quoted on the basis of the cheap ACM cladding.

[bluGill]

In a small house you can move outlets. In a large building the space for every wire and pipe is accounted for. Moving the outlet means that the wires to it have to move and in turn that means a plumber later can't come in and put the pipe where it must go. In a house they solve this by having the plumber come in before the electrician (the order is much more complex), but in a large building you need them working at the same time or you will never get done. Even in a house you still need to plan the plumbing as often a pipe and a beam need to go through the same place - either the beam moves or the toilet moves.

[michaelt]

Sometimes - but some large buildings have loads of space above suspended ceilings and in cable trays and suchlike. Office buildings know every new tenant might want to move internal walls, rearrange sockets, reconfigure the kitchen and so on, and they're built with that in mind.

You're right that making arbitrary ad-hoc changes during construction can be expensive, of course. Especially if it's wrong and has to be torn out and redone.

But it's not illegal - if you're in charge of a project and an installer tells you the specified vents are going to be a problem and they recommend a bigger one, nothing in law prevents you from telling them to go ahead and make the change, right there on the spot.

Of course, how often the person with in charge of a large project will be talking to the vent installers while they're working is a different matter...

[bluGill]

Many buildings are designed that way. It is a useful thing to do, but there are trade offs - you pay for that extra space in other materials and extra time. For an office the office/cubes/open layout is installed by the renters after they lease the building and thus the space you are talking about isn't part of the initial construction effort. They still plan/reserve space for the pipes and large wires in initial construction. Generally the bathrooms and break rooms (kitchens/cafeteria) would be done in initial construction and so carefully planned out.

It of course isn't illegal to make changes - but it is expensive and thus not something you should do. (though some things are much easier than others to retrofit half way in.

[lhnz]

I don't think you disagree with me. For larger projects, like infrastructure projects, very detailed designs are used and contracts that disallow substitutions or alterations to the design without contractual agreement.

I'm talking with some level of experience here. I did a residential project that was obviously much smaller than this but still very expensive (£100+k). Even for this architects recommended a detailed design and a contract that enforced this.