I think it happens to everyone, for sure if you have an old house. The architect does their best to estimate where the existing beams, pipes, and floor joists are… but they can’t always tell and old plans don’t always have that info. When they opened up the walls in our house they beams were not where we expected and the last remodel and done something funky by stacking the beam on the top of the floor joists (or something like that) which meant that now the floor joists for the new upstairs room needed to be twice as tall in order to match. This type of change requires us to submit a plan change to the building department. This is what we had to do:

1. The contractor, architect, and the structural engineer discuss the situation and make a plan to correct for the issues. (Luckily, in this case, they were super fast and there was a straightforward, not too expensive, alternative.)
2. The structural engineer and architect produce a new plan sheet and calcs.
3. Someone (usually contractor–but I went as well) takes the revised plan sheets AND new CALCS to the building dept (we forgot the the calcs the first time… which means two trips–ugh). Apparently sometimes you need a signed letter describing why you are making the change… depends on the building inspector you get.
4. The building reviewer at the city opens a plan revision and reviewed our changes. We had to pay a fee of around $100. Luckily this was simple enough that they are able to give you an “a-OK” stamp right there over the counter. (You don’t have to want 2 weeks for a review.)
5. Then you get back to work executing the new plans.

Wondering how many more of these we will have to do…