With a building that has a timber foundation, sitting on piers etc, you can easily make alterations to your original house cell as your needs change. If, however, you decide you need more room than you originally planned, and you have put your original cell on a full foundation, whether it be a slab or full basement, you will run into problems.
Additional rooms attached to your original house will have to have a foundation joined to the old one. It would be difficult to add to a foundation seamlessly. You will likely have air infiltration, a seam in your slab etc. Plus, a full foundation will probably be better if you can move from one part to another, meaning that you will have to cut a doorway into the old foundation.
The best way to avoid growth problems is to be realistic about your needs in the beginning. Young people need to amuse themselves in long winter months which sometimes leads to additional members of the family, grandchildren come to live with you, children move back home or you need a caretaker or caregiver, etc.
There are several ways to head off or cope with this eventuality.
Consider moving some of your activities to a new building, freeing up space in the old building. You may have built a studio or office in your original house. Move it to a separate structure, which may need only a minimal foundation and few amenities. Minimal electric service, no bath etc.
An entirely separate house for In-laws or children. Keep them out of your hair by building a new building from scratch. This will mean that you have thought about property line setbacks, septic systems etc in advance. Planning for the eventuality of adding a building, and siting your original in such a way that you will not have conflicts will save you lots of headaches.
Design your original house with a summer kitchen, porch etc. that shares your original foundation. This will allow you to simply upgrade the structure for full living space.
Design in pavilions. Plan on a central building, with dependencies in a logical arrangement around it. Perhaps covered walkways will connect guest suites, or board walks will wander through the woods to private accommodations. A bridge may lead you across a gorge or stream to another building. Again, setbacks will have to be kept in mind when you make your original plan. Keep in mind too, that zoning, property line setbacks etc. may change over time. Leave plenty of space, especially if your lot is not large, or the population is growing rapidly in your area. This may force your town to rethink their plan.
Your little house may be necessary for a time when money is tight, but as your needs change and money issues lessen, you may add buildings to eventually become a large compound.
One of my favorite memories is a retreat that I attended as an art teacher in Maine. A large group headed for Haystack on Deer Isle.
Glacier scoured granite islands have a thin layer of soil on them with wind stunted fir, spruce and pine crowning the level areas and clinging to the crevices down cliffs that descend steeply into the sea. Beneath them is a forest floor thick with ferns, mounds of peridot and emerald green moss, lichen and blueberries.
A main building and several studio buildings occupied the high ground that had most of the flat land on the site. A large deck occupies a central position with a flagpole and benches. From the deck, a wide staircase, broken by landings descends down the granite slope toward the sea. A couple of landings down, boardwalks turn off to either side, snaking through the woods. Along the boardwalks were tiny guest cabins, and one shower and bath. There were several of these side path levels, each lower down the ever steeper hill. At the bottom, the stairs ended in a landing, from which you could clamber down to the water. Icy cold in October, when we were there, it was a rite of passage to go down to the sea and skinny dip in the frigid water in the wee hours of the morning.
Everything shrank of course. My room-mate at the time met me on my way back to our room and poked me in the arm and hip. "You skin," he said.."its all....consolidated." Indeed, I bet I could have fit into a much smaller outfit after a dip in that water.