What to call it, and what the code calls it

Terms that appear in code

ADU

Short for accessory dwelling unit — the statutory term both Washington (HB 1337, RCW 36.70A.681) and Colorado (C.R.S. 29-35-401–405) use to authorize a second home on a single-family lot. Every city ordinance in this ledger is built around this exact term, whether the unit is attached, detached, or built inside the existing house.

In the code: All 28 cities in this ledger, Washington and Colorado alike.

DADU

Detached Accessory Dwelling Unit — a separate structure, as opposed to one built inside or attached to the main house. Washington cities treat DADUs as their own category with distinct height and setback rules. Colorado codes almost never use the abbreviation, writing "detached ADU" instead.

In the code: All 16 Washington cities in this ledger. In Colorado it appears only on our Colorado Springs page, informally — no Colorado code in this ledger uses it.

garage conversion

Converting an existing garage into livable ADU space rather than building new. Most cities address the method without naming it. Colorado Springs is the exception: its code sets a distinct 25 ft height standard for garage conversions, separate from the 16 ft cap that applies to other detached ADUs.

In the code: Colorado Springs

tiny home on wheels

A dwelling built on a trailer chassis rather than a permanent foundation, normally treated as an RV rather than housing. Redmond is a genuine exception: its code names tiny homes as an allowed ADU type, capped at 400 sq ft. Colorado cities go the other way — Westminster requires a permanent foundation and expressly excludes tiny homes on wheels, while Colorado Springs allows prefab and tiny homes only on permanent foundations.

In the code: Redmond (allowed, 400 sq ft); Westminster (excluded); Colorado Springs (allowed only on a permanent foundation)

middle housing

Washington's state-law term, from HB 1110, for duplexes, triplexes, and similar small multi-unit buildings. It is a separate legal category from ADUs, but the two interact: several cities cap ADU counts differently on a lot that already has middle housing. The term does not appear anywhere in this ledger's Colorado corpus.

In the code: Bellevue, Bothell, Burien, Edmonds, Issaquah, Sammamish

unit lot subdivision

A Washington mechanism that lets an owner subdivide one lot on paper so an ADU can be sold separately, like a townhome, without a full physical split. It is the usual answer to "can I sell the ADU?" in Washington. No Colorado code in this ledger uses the term.

In the code: Bellevue, Everett, Kirkland, Olympia

owner-occupancy

A requirement that the property owner live on site — historically the default rule for ADUs, now mostly abolished. Every city in this ledger addresses it, almost always to say it is no longer required, since both states barred ongoing owner-occupancy mandates. Holdouts remain: Arvada and Thornton still enforce it by recorded deed restriction, which the state flagged in October 2025.

In the code: Addressed explicitly in all 28 cities.

Terms people use that the code doesn’t

These are perfectly good English. They just have no legal meaning — searching a city’s code for them returns nothing, which is why they can be hard to research.

AADU

Attached Accessory Dwelling Unit — the counterpart abbreviation to DADU, for a unit built onto or within the main house. It is common zoning shorthand nationally, but it is worth knowing that no city in this ledger uses it: all 28 write "attached ADU" in full, even while treating attached and detached units very differently on size, height, and floor-area rules.

In the code: not used by any city in this ledger.

backyard cottage

A plain-English nickname for a detached ADU. It appears in our own explanatory language for Seattle and Bellevue, but never tied to a code citation — the ordinances in both cities call the same structure a detached ADU or DADU, never a cottage.

In the code: not used by any city in this ledger.

granny flat

A common informal name for a small secondary unit, usually meaning the same thing as an ADU. No Washington or Colorado code in this ledger uses this phrase — the word "granny" does not appear anywhere in the 28 municipal codes reviewed here. Every city instead defines and regulates an "accessory dwelling unit."

In the code: not used by any city in this ledger.

mother-in-law apartment / in-law suite

Popular shorthand for a self-contained secondary living space, often inside or attached to the main house. No code in this ledger uses "mother-in-law" or "in-law" in any form. The legal category covering the same space is an attached or interior ADU — and it is regulated identically regardless of who lives in it.

In the code: not used by any city in this ledger.

casita

Spanish for "little house," and the dominant everyday word for a detached ADU across much of Colorado's Front Range — but it carries no legal weight. No city code in this ledger, including Denver, Boulder, and Colorado Springs, uses "casita" anywhere. All of them regulate the same structure as an accessory dwelling unit.

In the code: not used by any city in this ledger.

carriage house

Historically a structure that housed carriages below and living space above; now used loosely for any garage-topping ADU. It is not a regulated category in this ledger. Olympia uses it as the name of one pre-approved stock plan (a carriage-house-over-garage design), and Denver's code has a related but different idea — a "carriage lot" ADU, which may exist with no primary dwelling at all.

In the code: Olympia (as a stock plan name); Denver (as "carriage lot," a distinct concept)

guest house

An informal term for a small detached structure meant for visitors rather than a full second household. Bellevue comes closest to giving it legal footing: LUC 20.20.120.C.3 exempts guest-cottage conversions from the ADU size cap. That is a distinct phrase, and the only brush with a legal term anywhere in this ledger. No city uses "guest house" itself.

In the code: Bellevue (as "guest cottage," a related but distinct phrase)

basement apartment

Everyday language for a unit built into an existing basement. Legally this is one construction method for an interior ADU, not a separate category. Several codes here address basement units directly — Longmont exempts basement ADUs from its size cap entirely — but always as an ADU, never as an "apartment."

In the code: not used by any city in this ledger.

Term usage is checked against the same verified city records as the rest of this site; see methodology. If a city adopts one of these words into its code, it will show up in the changelog.