I really wish a company would make modular homes that are actually modular, in that you could start with a 'tiny home' module, then a few years down the line add a 'bedroom and master bath' module and turn the original bedroom into a child's space, and so on. The end result might be at a premium over getting a big house all at once, but in the meantime you'd get the benefit of being able to invest most of your money instead of sinking it into a mortgage.
There'd be a lot of waste with that with pipes and electrical connections leading no where and at best lots of extra framing around the non existing doors that would be cut out to install the new section. There's also the issue of how to deal with the roof and cladding that would have to be sealed completely between the two sections against water and pests. It's doable but there's a lot of unavoidable work that'd have to be done to connect the two bits properly on top of whatever additional site preparation to extend the footprint of the foundation.
That work would be a big portion of the cost of adding on to a house much like additions are made now.
It would probably need to be constructed from standalone modules with something akin to an airlock to get between them. Even then whatever you save by buying piece-at-a-time might still end up more expensive than just building a bigger house and blocking off parts until you need them.
