It’s common to analogize writing software with building a house.
I think it’s a lot more like writing a novel.
With a house, you can add an addition and not necessarily have to change much. But with a novel, everything in it has to reconcile.
The father becomes a mother
Imagine a novel about father who has a troubled relationship with his son because the father spends too much time at the bar, getting in fights and getting fired from a string of jobs due to his irresponsibility. In the end the father dies in a drunken car crash and, fast forward, the son grows up to be just like his father. (Sorry, didn’t mean to make it so dark!)
But then imagine “the requirements change” somehow and the father character needs to be a mother instead.
We can’t just search-and-replace “dad” with “mom”. All sorts of other things need to change as well if the whole thing is going to square up.
If the tragedy of the story is that the son turns out to be just like his father, then maybe a mother-son thing doesn’t really make much sense anymore. Maybe the son has to be a daughter instead. A mother can just as plausibly be an alcoholic as a father of course, but the part about the constant fist fights makes a little less sense now. That part will probably either have to be eliminated or changed to something else.
Every codebase is a novel
Software is of course the same way. Some changes are isolated sometimes, but quite a lot of changes have ripple effects throughout the whole system. And the bigger a codebase gets, the more challenging and time-consuming it is to keep everything self-consistent.
This is why the construction analogy doesn’t really fit all that well. I think the novel analogy is better.