Three things about Princeton properties that change how projects get planned, and that homeowners are often surprised by.
Almost everything here is on private septic and a private well rather than town sewer and water. Massachusetts sizes septic systems by bedroom count under Title 5, which means any project that adds a bedroom, including most additions and nearly every ADU, has to be checked against your system's rated capacity before design starts. If the system cannot carry it, the project becomes a septic upgrade plus an addition, which is a different budget entirely. Well capacity matters for the same reason when you are adding bathrooms.
Princeton also has its own municipal utility, the Princeton Municipal Light Department, rather than one of the large investor-owned utilities. If your project needs a service upgrade, and older Princeton homes frequently do once you add a modern kitchen or a second unit, that goes through PMLD. Knowing how that process works and how long it takes is the kind of thing that keeps a schedule intact.
Third, lots here are large and many are wooded, sloped, or wet in spring. Site access, drainage, and Conservation Commission review near wetlands are all real considerations on Princeton properties in a way they are not on a quarter-acre lot in the city.