First, you might get a better answer if you provide more info. Like any details on the German ancestors, if you know them. What type of house this is, if there is any sort of an architectural style and where it’s located.

Based on what you provided, it really depends a lot on how you look at the question. If I was a man living in the 18th or 19th century given the choice between building a new home or building an addition, it would be an easy choice, the addition is better because it's cheaper and less labor intensive.

Still are there examples of people building temporary medium term housing? Yes, of course. The Pilgrims for example are a common example of a group that largely built temporary homes and then built permanent ones later on. However, there were two key factors in their motivation for doing so. 1. They were extremely desperate and had no shelter when they landed, so they hastily built everything. 2. The colonial government basically mandated that they build the homes within the village on so-called "home lots". The agricultural plots were outside of town and often not even connected to the "home lot". Once the colony got going people gradually moved to the agricultural plots and built homes there. So there are examples of people living in say one spot for a decade or two and then building a brand new home nearby, but there was a fair amount of desperation involved and a set of circumstances that did not exist in many other places and times.

You've noted that these ancestors might be German and from upstate New York. But without knowing exactly who these people were, where they settled and what specific migratory group they were part of it may be hard to say much about the likelihood of them building temporary or semi permanent housing. For example the 18th century German Palatines were effectively rather impoverished religious refugees. While the 19th century German immigrants were usually skilled workers. So these groups likely had very different patterns in how they built their homes.

There is a rather interesting article on JSTOR called "Their Houses are Ancient and Ordinary": Archaeology and Connecticut's Eighteenth-Century Domestic Architecture, by Ross K. Harper. It skews a little earlier and is not in New York, but it does go into some of the common situations. Many homes had several additions done to them provided they remained in good condition. There are reasons a home that existed in 1787 might not exist in 1840, like a fire. You also have to consider that in many agricultural families they’re going to want to pass on land to heirs. However, depending on the number of children and who needs a house at what point in time. You can very easily end up in situations where at one point in time you don’t have enough houses for everyone in the family, so you build a new one. Or other points in time, usually after someone passes away where you may have an extra house and not know what to do with it.

The only other point I would make is that you said you’ve seen the survey data for 1787. If the survey data in any way provides an indication of size that might tell you something. I would be a little surprised if there was a large home in a given location, it was completely demolished and then another built in effectively the same exact spot in a 50 year timespan, but that is speculation.