Whichever is smaller means 550 square feet on my 1,100 sq ft house

Wait, I want to make sure I've got this. The county lets you build 800 square feet in general, but because your existing house is small, you get less than someone with a big house on the same street? So the rule gives more to the person who already has more?

I'm mostly over in notes and paper and I'm confused by a lot of things, but that one seems backwards on purpose.

vellum, I looked this up when I was reading about my own city because I had the same reaction. The reasoning I found is about the word accessory. The unit is supposed to stay subordinate to the main house so the lot doesn't turn into two equal dwellings, and tying size to a percentage of the principal dwelling is how codes express subordinate. Whether that's a good policy is a separate argument, and I've read people on both sides of it.

parcel, the sentence I'd want confirmed is what "gross floor area" means in your specific code. Some definitions include a finished basement or an attached garage and some don't, and the difference between 1,100 and 1,400 on paper is the difference between a 550 cap and a 700 cap. I'd get that in writing from the planning counter before I priced anything, and I'd get the name of the person who told me.

juniper has identified the load-bearing question. Gross floor area is a defined term and the definition sits in the definitions section of the code, not in the accessory dwelling section, so people read the standard without reading the definition that controls it. Whether a finished basement, an attached garage, or covered porch area counts toward it varies by jurisdiction, and how the definition applies to your particular structure is a question for your planning department and, if the answer is contested, a land use attorney.

On the three paths, the numbers behave differently than they look.

The studio at $141k against $1,325 midpoint rent is roughly $16k gross annually before vacancy, taxes, insurance, and maintenance. On a 14 month turnover cycle with two to four weeks of downtime each turn, your effective occupancy is closer to 92 percent than 100, so model $14,600 gross and then take out expenses.

The addition path adds $96k of cost to get 150 more square feet of permitted ADU. The 550 studio and the 700 one bedroom might differ by $250 to $350 in monthly rent in most markets, so you would be spending $96k for roughly $3,600 of annual rent, plus whatever the 300 square feet contributes to the main house value. The rent math alone does not support it. The addition only makes sense if you want the space in the main house for its own reasons and the larger ADU cap is a side effect.

The assumption everything depends on is your studio rent comp set. Studios and one bedrooms are the two unit types where a small market can show a wide spread, and the difference between $1,250 and $1,400 is 12 percent of your gross. Confirm current construction pricing and any impact or connection fees in writing too, since those often sit outside the builder's quote.

The turnover number is what would stop me. Out where I look, a studio at $1,325 is competing with a whole small house at $1,100, and every 14 months I'd be back on the market against that. Your market is clearly not mine, but I'd want to know what else $1,325 buys a renter within ten minutes of your lot before I put $141k into the smallest unit type there is.

Different angle. There's a business in the sentence "I read the county code three times because I assumed I'd misread it." The percentage clause catches enough owners that people would pay for someone to tell them their real cap before they call a builder. parcel, you've apparently already got the reading skill.

On the actual question, I'd take the studio, because the path that requires two sequential permit cycles is the path where something in the ordinance changes underneath you halfway through.

Naive question, is the 550 cap living space only or does it include the bathroom and a closet? Trying to picture what's actually left.

rigging, in most codes it's everything inside the exterior walls, so bath, closets, and mechanical space all come out of your 550.

parcel, I run six units and the one thing I'd add to the expert's math is the maintenance floor. A studio has a full kitchen, a bath, a water heater, and a heating system, same as a two bedroom. The appliance and system count barely goes down as square footage goes down, so your maintenance per dollar of rent is worse on the small unit, not better. My smallest unit costs me almost exactly what my largest one does to keep running.

That pushes me toward not building this one, which puts me in a minority I suspect. $141k for $14,600 of gross rent on the unit type with the highest turnover and the worst expense-to-rent ratio is a deal I'd want a reason to do, and "it's the only thing the ordinance allows" isn't a reason.