The lesser-of clause in ADU ordinances often cuts the real buildable size in half

Question

Many ADU ordinances cap floor area at the lesser of 800 square feet or 50 percent of the gross floor area of the principal dwelling. On a 1,100 square foot principal dwelling, that clause caps the accessory unit at 550 square feet, not 800, a studio or a very tight one bedroom if the design is efficient. That percentage clause is common and it catches people who read only the flat square footage figure that news coverage tends to quote. The flat cap is the number that gets headlined. The percentage clause is the number that actually governs most lots, and it varies by county and city even within states that have opened ADU policy broadly, so the only version that matters is whatever is currently on the specific jurisdiction's books. Three paths tend to follow from a constraint like this. Building the smaller studio outright, at a quote around $141k for a detached unit with a full kitchen and a three-quarter bath, against studio rents of $1,250 to $1,400 and turnover roughly every 14 months, is the shovel-ready option. Adding to the main house first to raise the 50 percent ceiling, say 300 square feet at roughly $96k, pushes the cap toward 700 square feet but produces no rent on its own and means running two permits and two builds in sequence. Holding the lot and waiting on ordinance changes is the third path, though percentage clauses tend to be the last provision that moves in an ordinance rewrite, so that wait can run for years. The percentage clause is worth checking against the actual current ordinance before pricing anything, since it's the single most common source of a buildable-size surprise in this niche.

raj_mehta14 · Answer

To make sure the reading is right: the county allows 800 square feet in general, but because the existing house is small, the owner gets less than a neighbor with a big house on the same street. So the rule gives more to the person who already has more. That is the mechanism as written, and it is worth saying plainly because it looks backwards on purpose to anyone meeting it for the first time.

kevin_lombard · Answer

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.

Rena Expert · Answer

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.

laurenkim53 · Answer

The turnover number is the one to stop on. In plenty of markets a studio at $1,325 competes with a whole small house at $1,100, and every 14 months the unit is back on the market against that. Before putting $141k into the smallest unit type there is, find out what else $1,325 buys a renter within ten minutes of your lot.

derekj19 · Answer

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.

megan_doyle15 · Answer

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.

brandonlee36 · Answer

Rigging, in most codes it is everything inside the exterior walls, so bath, closets and mechanical space all come out of your 550. Parcel, the one thing to 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, the same as a two bedroom. The appliance and system count barely drops as square footage drops, so maintenance per dollar of rent is worse on the small unit. On a small portfolio the smallest unit typically costs almost exactly what the largest one does to keep running. That argues against building this one, which is probably a minority view here. $141k for $14,600 of gross rent on the unit type with the highest turnover and the worst expense to rent ratio needs a reason, and the ordinance allowing nothing else is not one.

The lesser-of clause in ADU ordinances often cuts the real buildable size in half

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