[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.