ADU (Accessory Dwelling Unit)

An ADU is a second, legally independent home on a lot that already has one. For most people reading this site it arrives from the wrong direction: you build a backyard bar with a fridge and a TV, then think about adding a bathroom so nobody has to trek back to the house, and somewhere in that sequence the building stops being a shed and becomes a dwelling unit in the eyes of the planning department. Detached new-build, garage conversion, basement conversion and attached addition are all ADU shapes, and California additionally recognises a junior ADU carved out of the existing house.

The distinction is worth understanding before you pour a slab, because the two sides of that line are regulated nothing alike. An accessory structure gets a straightforward permit, or under a size threshold sometimes none at all. A dwelling unit brings plan review, energy code compliance, egress requirements, fire separation, a permitted electrical feeder, a plumbing connection, smoke and carbon monoxide alarms, and a property tax reassessment on the value you just created. The costs follow: a detached ADU in California runs roughly $300 to $500 per square foot turnkey, landing most projects between $225,000 and $400,000 as of 2026, while a shed conversion sits an order of magnitude below that.

What an ADU is not is a general term for a nice backyard building. A structure with a wet bar, a beer fridge, a sofa and a game table or arcade cabinet is an accessory structure however much you spent on it, because it has no permanent cooking, sanitation or sleeping provision. Rules also vary enormously between states and cities, and California, which has the most developed ADU law in the country, is a poor guide to what your own jurisdiction allows.

The five-facility test

California Government Code section 66313(a) carries the clearest statutory wording on this anywhere in the country. It defines an ADU as a unit providing complete independent living facilities, with permanent provisions for living, sleeping, eating, cooking and sanitation. Five functions, and the load-bearing word is permanent.

Read against a backyard man cave, that gives you a workable test. A bar sink and a beverage fridge are not a cooking facility. A microwave on a shelf is not one either. A fixed range or cooktop with counter space and food storage is, and a bathroom with a toilet, sink and shower is sanitation. Add a space set up for sleeping and you have the full set, at which point the building is a dwelling unit whether or not anybody filed it as one.

That threshold matters because of what sits on either side of it. IRC section R105.2 exempts one-story detached tool and storage sheds from a building permit up to 120 sq ft in the base code text, with some jurisdictions using 200, and that exemption is where most backyard projects begin. It never covers trade work. Run a circuit or a water line out to that shed and the electrical and plumbing permits apply regardless of the building’s footprint. Cross into dwelling-unit territory and the rest arrives at once: a seven-foot minimum ceiling height for habitable space under IRC R305.1, an emergency escape opening of 5.7 sq ft net clear with a sill no higher than 44 inches in any sleeping room under R310, energy code compliance, and fire-rated construction if you sit close to a property line.

What crossing the line buys, and what it costs

Going through the ADU process is not purely a burden. A permitted second unit is rentable, financeable and disclosable at sale, and mortgage underwriting now recognises it: Freddie Mac allows up to 75 percent of documented market rent from an ADU to count toward qualifying income. An unpermitted backyard build does none of that, and it surfaces at exactly the worst moment, during an appraisal or an insurance claim.

The costs are real in the other direction. California exempts ADUs under 750 sq ft from local impact fees and charges proportionally above that, but permit and plan-check fees still run into the thousands, design and engineering soft costs land somewhere around $9,000 to $30,000, and utility connections reach $35,000 on difficult sites. Property tax goes up, though only on the new construction, since Proposition 13 protects the base-year value of the existing house and the assessor adds the ADU’s value on top.

The honest read for most man cave projects is that you do not want an ADU at all. You want a well-built, properly permitted accessory structure with power, insulation and no shower, which costs a fraction as much and answers to a much shorter list of rules.

Where the term came from

The building is far older than the acronym. Granny flat, in-law unit, casita and ohana all describe the same thing, and Hawaii has allowed a second dwelling on single-family lots since around 1981. Accessory dwelling unit is the planning-department name, and it spread because states needed a defined term to write legislation against.

California moved first at scale, with statewide ADU legislation in 2016 and repeated strengthening since. In 2024 the whole scheme was renumbered, so the Government Code section 65852.2 that older guides cite became sections 66310 to 66342, effective 2025. Another round of bills was signed on 10 October 2025, including SB 543, which reaffirmed the 500 sq ft cap on junior ADUs. Other states have followed on their own timelines, among them Oregon, Washington, Montana, Maine, Utah, Colorado, Arizona and Massachusetts, whose Affordable Homes Act ADU provisions took effect in February 2025. Counts of how many states now have by-right ADU law vary between roughly twelve and eighteen depending who is counting, which tells you how quickly this is still moving.

Three ways backyard builds go wrong

The bootleg circuit. Running a cable from a spare breaker in the house out to the shed, unpermitted and usually undersized, is the most common failure in these projects. It is a code violation, it is a documented reason for an insurance claim to be denied after a fire, and it is the first thing an inspector notices. Pull the permit, size the feeder for the real load, and if the room will hold a fridge, an amplifier and a projector, plan that load properly before the trench goes in.

Adding a shower without meaning to. People fit a bathroom for convenience and unknowingly complete the five-facility set, converting a legal accessory structure into an unpermitted dwelling unit. If you genuinely want a bathroom out there, go through the process deliberately and permit it as what it is, rather than discovering the problem during a sale.

Skipping insulation and moisture control. An uninsulated backyard building is a three-season room, and in a cold or humid climate it grows mould in the walls. Insulate walls, ceiling and floor, detail the vapour control for your climate zone, and check septic capacity before adding any fixtures if you are not on a sewer, because a system sized for the house decades ago may have nothing spare.