Some regulators have different rulesets for 'temporary structures' (whose permanence can be fuzzy), for 'extensions' (whose connectedness can be limited), or 'renovations' (whose alteration vs. new-build can be gamed). Work the system. Worst case, put up a glamping tent and go neo-Mongol.
On the glamping line of thinking, I've seen some families around here use an RV as a mother-in-law apartment. They are a bit expensive right now, but still way cheaper than full renovation to add the same amount of space. Plus, then you have an RV.
