I'm literally doing this right now.
We're buying a house, our first.
The only people that know;
My aunt and uncle
My boss and a colleague
His boss and some colleagues.
We've not told anyone else. None of my siblings, none of our parents, no friends. Oh the landlord he knows (a friend).
Why?
1/. In case it falls through, it'll be difficult enough for us emotionally with us having to listen to bullshit pithy phrases
2/. Don't really like any of our parents
3/. Don't want needless input. Demanding we have a South facing garden/this survey/etc. Etc. Because they needed those things, no thought as to what we require
4/. Don't need them prying into our finances
5/. Ultimately it's none of their goddamn business.
We'll tell them when it becomes necessary - probably around completion.