Also as a tip in future here OP, don’t let things be open to any sort of negotiation right from the start.
For example you say...
I’m building an extension for ME
See it’s a statement, a declaration NOT a question, not a discussion, it’s just is what it is.
What’s happening now is that you’ve essentially said....
I’m building an extension for more space and then later figure out who will get the benefit of it.
It’ should simply not be up for discussion, you are the adult, you are the one paying for it, it’s YOURS and that’s the end of conversation right there!!!
Does moaning a whining get you you the corner office and that big fat promotion and pay rise at work?? Absolutely fucking not....and why??? Erm well because you have to work for it, you have earn it, no one is ever going to hand you something just for whining and kicking up a huge fuss about it. Except that’s exactly what your children WILL expect, because it’s how they are behaving towards you already. As I said before IF this continues they are in for a hard life as adults, because you cannot just get your own way in the workplace, you cannot just get your own way in friendships/relationships, not in healthy ones anyway and I am sure as the loving and caring mother you are want them to have healthy relationships. Healthy relationships start with healthy firm boundaries and boundaries are necessary for us all, as children and as adults.