But you also have to keep in mind that those who put things off until they "had space" or were "financially far more secure" are now often posting on the "trying to get pregnant in late 30s - is it too late" type of threads.
Realistically speaking - how many people keep the pushchair and the highchair and such in the bedroom for heaven's sake. As far as bedrooms are concerned, it's a cot and a dresser. We keep our pushchair in the boot of the car, it's there when we need it, but out of the way. We have a playpen, but while it's nice, it's certainly not a necessity - lots of people don't have them or don't use them. And that's only for a very short time. By the time they're two or so, the pushchair, the playpen, the highchair can all be gotten rid of. It's temporary.
Living space includes living room, bedrooms, dining room if you have it, kitchen, and rear garden (if you have one as well). Two kids in a flat is certainly not an urgent need to relocate to a house with a garden, if you're happy in the flat. There are parks and other places you can take the children - lots of people function without their own garden.
It's not just a matter of space, but a matter of the future as well. If you have any concern about whether or not you'll have trouble conceiving when you're older and you definitely want to have another, keep in mind that it will possibly be much more difficult to conceive once you hit your late 30s. For some it isn't (we have a 26yo, 24yo, 4yo, and 1yo) but for some it is. And that's certainly something to keep in mind.
I just hate seeing people act like a child not having it's own room or a garden is tantamount to neglect or not providing for your child. It's just nonsense.