ADU vs addition: which adds space better in California?

ADU vs addition at a glance

An addition makes your existing house bigger. An ADU is a separate dwelling, attached or detached, with its own kitchen, bathroom and entrance; California law defines it as a unit with “complete independent living facilities.” That legal difference drives almost everything below. If you're new to the term, see what an ADU is.

Sources are linked in the sections below.

Permits: ministerial ADU vs addition permit

The ADU permit

An ADU application gets ministerial review: the city checks it against objective standards and must approve it if it complies. Under Government Code section 66317, the city has 15 business days to tell you whether the application is complete and 60 days from a complete application to approve or deny it. If it misses the deadline, the permit is deemed approved. Plans a city has pre-approved get a 30-day review.

The state also limits what a city can demand. It can't deny an ADU permit because the property has nonconforming zoning conditions, such as an older house that sits inside today's setback, and it can't require parking within half a mile walk of public transit. In the City of San Diego you apply through the Development Services portal; our San Diego ADU permit guide walks through the steps.

The addition permit

An addition is a regular residential building permit. It still goes through plan check, but none of the ADU protections apply: no 60-day deemed-approved deadline, no cap on setbacks, no guaranteed size. The addition must meet every rule of your residential base zone, including setbacks, height and floor area. The city posts current review times for all permit types on its permit timeline page, so check it before you plan a schedule.

Setbacks, height and size

This is where an ADU can fit when an addition can't. For a new ADU, a city can't require side or rear setbacks of more than 4 feet, and converting an existing structure needs no setback at all. San Diego's ADU code goes further: an ADU 16 feet tall or less needs no interior side or rear setback, except 4 feet in high or very high fire hazard zones.

An addition gets none of that. It must respect the base zone's front, side and rear setbacks, which in San Diego's single-family zones are set in the zone tables of Chapter 13 Look up your zone before you sketch anything.

Sources: Gov. Code § 66321 for size, height and the 800-square-foot guarantee; SDMC § 141.0302 for San Diego's ADU sizes and setbacks. If your house already uses most of its allowed floor area, an ADU may be the only way to add living space at all.

The ADU addition: an attached ADU

An “ADU addition” is an attached ADU: new space built onto your house that is a separate unit, with its own kitchen and entrance. It gets the ADU setbacks and permit clock, but it isn't on the state's short list of guaranteed types, so it follows the city's ADU rules. It also needs a fire-rated separation from your house. The attached vs detached ADU guide covers the details, and two-story ADUs covers building one over a garage.

Fees

Impact fees. An ADU of 750 square feet or less pays no impact fees; a larger one can be charged only in proportion to the main house. The City of San Diego exempts the first two ADUs on a lot from its development impact fees regardless of size. Those protections are written for ADUs; for an addition, ask the city which fees apply to your project.

School fees. Here the two are treated alike. A school district can charge on an addition only if it adds more than 500 square feet of assessable space, and once it does, the fee applies to the whole increase. ADUs under 500 square feet are exempt too. San Diego Unified charges $5.38 per square foot, so a 600-square-foot project pays about $3,228.

Permit fees. Plan check and permit fees apply to both. Snap ADU's February 2026 table puts total City of San Diego fees for an ADU at $10,734 at 499 square feet and $25,138 at 749 square feet; those are a builder's figures, not an official schedule. For additions in unincorporated areas, the County's 2025/26 fee schedule lists plan review at $2,111 plus $0.358 per square foot and the permit at $1,395 plus $0.722 per square foot for an addition over 100 square feet that needs plan check. Block Renovation estimates that plan check, permit and any impact fees add $5,000 to $15,000 to a San Diego addition.

Utilities and solar. A detached ADU may need its own utility runs, and the Energy Commission says a newly built detached ADU generally needs solar panels. Attached ADUs are treated as additions under the Energy Code, so solar isn't required for them.

Property tax

Property tax is a tie. Under California's definition of new construction, any addition to real property counts, and only the newly built portion gets a new assessed value. The rest of your property keeps its Proposition 13 base.

So an ADU and an addition are taxed the same way: the San Diego County Assessor adds the market value of the new construction to your existing assessment, and you get a supplemental bill once it's done. At the 1% base rate, every $100,000 of new assessed value adds about $1,000 a year, plus local bonds. What differs is the amount: a full ADU with a kitchen and bath is usually worth more than a bedroom addition, so its tax increase is usually larger.

Renting and resale

Renting. An ADU can be leased as its own home. The City of San Diego requires ADU rentals of at least 31 consecutive days. HUD's 2026 fair market rent for a one-bedroom in San Diego County is $2,459 a month. A home addition is part of your house; it can't be rented as a separate dwelling because it isn't one. See ADU rental income in San Diego for rents and tenant rules.

Borrowing. Fannie Mae lets lenders count rent from an ADU on a one-unit home toward your qualifying income, within limits. An addition doesn't produce income a lender can count.

Appraisal. Fannie Mae tells appraisers not to include an ADU in the main house's gross living area; it's listed and valued separately from comparable sales. An addition simply makes the house bigger, which appraisers already know how to price. An ADU can add more value where buyers pay for rental units, but with few local comparables it can get less credit.

Selling separately. The city's ADU code also allows an ADU to be sold as a condominium. An addition never can be. For more, see what an ADU means in real estate.

Cost per square foot

Per square foot, an addition is usually cheaper. A plain bedroom or family room needs no kitchen, may need no new bathroom and ties into the house's existing utilities. An ADU needs all of that.

Sources: Block Renovation's San Diego additions guide; Lars Remodel on bump-outs and on ADU costs; Snap ADU's published prices, which include design, permit management and level-lot sitework but not city fees. These are contractors' planning figures, not bids, and they define scope differently, so compare like with like. For ADU pricing on your lot, see what an ADU costs in San Diego or the ADU cost calculator.

Which to build

Choose an addition when the space is for your own household (a bedroom, a bigger kitchen, a primary suite), you have room within your zone's setbacks and floor area, and you don't want a second kitchen or tenants.

Choose an ADU when you want rent or a separate home for a relative, your lot is too tight for an addition under base zone setbacks, your house is already near its floor area limit, or you want the faster, deadline-bound permit.

Consider a junior ADU if you want a semi-separate space without building new square footage. It's up to 500 square feet within your existing house, with an efficiency kitchen. See the junior ADU guide, or weigh the bigger picture in ADU pros and cons.

Either way, get bids from licensed builders. Our verified ADU builders in San Diego all hold an active B license, and you can check a contractor's license before you sign. Our guide on how to hire an ADU contractor covers deposits and contracts, which follow the same home improvement rules for both projects.

Common questions