You can only look so far ahead when you purchase a home. It’s impossible to tell when a new baby might become an unexpected family addition and for many families, the home they bought isn’t big enough a few years later once there are more people living in it.
If you find your home just doesn’t have enough space for the number of people living in it, you have a few choices. Moving to a new house seems a ridiculous step to take when you could just improve the existing home.
Rather than building out and taking up more land, your best bet then would probably be to build upward and into your attic. It’s actually pretty easy to make your attic a spare room which is as safe as the rest of the house.