If you want people to have kids, you need to make sure they are economically secure by the time they are of childbearing age... which means before they are 30. To do that, you cannot have supply constrained zero-sum shortage anywhere in society. It means that the cost of an apartment needs to be at-or-below the cost to build that apartment so people can just save up and buy one early on in their career.
We need to not just allowing housing to get built, but actually we need to go as far as subsiding housing that nobody needs so that it's built before that need ever arises.
This is effectively impossible in a democratic society, and we are going to learn just how impossible it is as western society slowly collapses under the weight of its own social programs. It's honestly horrifying to watch.