I agree with Katja: if just one more window already disrupts the financial plan, then you should really consider whether you are overextending yourselves with the construction. Usually, it ends up being more expensive than originally planned (so a side question: is the financing already secured? Have you planned enough buffer? Not everything can be covered with EL!).
And then all of this on a sloped plot, where the earthworks will not be insignificant. I don’t want to spoil it for you, but please calculate carefully and be realistic and conservative.
You are still young, probably planning to have children -> also factor in the loss of income.
I really sound like a party pooper times three here, but as I said, if the idea of designing the windows differently or just one or two more windows already brings you to the edge of what’s financially feasible, then I get a really bad feeling about this...