Boise ADU + Zoning Glossary

ADU (Accessory Dwelling Unit)

An Accessory Dwelling Unit is a self-contained secondary dwelling on a single-family lot. It has its own kitchen, bathroom, sleeping area, and exterior entrance, and is permitted as a dwelling — not as a shed, studio, or accessory structure. Under the City of Boise's 2023 Modern Zoning Code, an ADU is capped at 900 sq ft of finished living area (raised from 700) or 50% of the primary dwelling's living area, whichever is less, and is limited to two bedrooms. ADUs can be detached, attached to the primary dwelling, or built as a garage conversion. Owner-occupancy was removed in December 2023, so a Boise ADU can be operated as an independent long-term or short-term rental.

Source: City of Boise — Pre-approved ADU plans & code

Attached ADU

An attached ADU shares at least one wall with the primary residence. Common configurations include basement conversions, above-garage units, and rear additions with an independent entrance. Boise treats attached ADUs under the same 900 sq ft / 50%-of-primary cap as detached units, but attached configurations sometimes allow more total square footage because the primary-dwelling math can be more generous on larger homes. Attached ADUs typically build slightly cheaper per square foot than detached because the foundation and one or more exterior walls already exist. They also avoid a separate sewer lateral on most lots. Trade-offs: less rental privacy, shared mechanical systems, and (in some HOAs) more aggressive design review than a detached pre-approved plan.

BMC (Boise Municipal Code)

The Boise Municipal Code (BMC) is the codified body of city ordinances that govern zoning, building, licensing, and land use within Boise city limits. ADU regulations live in the residential section of the Modern Zoning Code (effective December 1, 2023). The BMC is the controlling document any time a planner, plan-checker, or inspector references a Boise-specific rule — it sits above administrative guidance and below state statute. The full code is hosted on American Legal Publishing's codelibrary, with the city's planning department linking authoritative sections from its own pages. When city staff cite a section number (e.g., 11-06-something), they are citing the BMC. Outside city limits, Ada County's ordinances or another municipality's code apply instead.

Source: Boise Municipal Code (American Legal)

Buildable area

Buildable area is the portion of a lot left over after subtracting required setbacks, recorded easements, and any overlay-driven no-build zones (hillside slope tiers, riparian buffers, historic-overlay impact areas). It is not the same as lot size — a 7,500 sq ft Boise lot with a 25 ft front setback, 10 ft rear, 5 ft sides, a utility easement along the back fence, and a primary residence already on it might have only 600–900 sq ft of usable buildable envelope. Buildable area is the single most important number on a lot check, because it determines which pre-approved plan footprint actually fits and whether the project needs a variance, a custom plan, or a different lot entirely.

Cap rate

Capitalization rate (cap rate) is net operating income divided by total project cost, expressed as a percentage. A 9% cap rate ADU produces $9 of net annual income for every $100 invested. Cap rate is the standard income-property valuation metric and lets a homeowner compare an ADU build against alternative real-estate investments on apples-to-apples terms. For a Boise Kingfisher 1BR rented at $1,650/month long-term against a $200k all-in build, gross yield runs ~9.9% — and cap rate, after subtracting taxes, insurance, vacancy reserve, and maintenance, typically lands 1.5–3 percentage points below gross. Cap rate ignores financing, which is why cash-on-cash return (a separate metric) is the right figure for leveraged builds.

Cash-on-cash return

Cash-on-cash return is annual pre-tax cash flow divided by the cash actually invested — down payment, closing costs, and any out-of-pocket construction spending not covered by the loan. Unlike cap rate, it accounts for financing. A Boise ADU built with a HELOC or construction loan at 70–80% LTV typically produces a cash-on-cash return materially higher than its cap rate, because rental income covers debt service plus operating cost and the remaining cash flow lands on a smaller invested base. The figure is most useful in year one and again at refinance; over a 10-year hold, total return (cash-on-cash + appreciation + principal paydown) is the better lens. Cash-on-cash math is sensitive to interest rate movement.

CC&R (Covenants, Conditions & Restrictions)

CC&Rs are recorded private deed restrictions that bind every lot in a subdivision. They sit on top of city zoning and can ban ADUs, short-term rentals, or specific exterior treatments even when the City of Boise allows them. The city does not enforce CC&Rs — only the HOA does, typically through architectural review committees and (rarely) legal action. Boise homeowners should pull their CC&Rs from the title file before committing to an ADU project; the document will spell out what the HOA can and cannot regulate. Common ADU-relevant CC&R clauses: minimum lease length (often 6 or 12 months), exterior material restrictions, separate-mailbox prohibitions, and outright bans on secondary dwellings.

Custom ADU

A custom ADU is designed from scratch (or significantly modified from a stock plan), as opposed to one of the City of Boise's six pre-approved plans. Custom typically adds $8,000–$15,000 in architectural fees and 4–8 weeks of design-review time at the city. It makes sense on unusual lots (steep, narrow, oddly shaped, or hillside-overlay parcels) and for use cases the pre-approved plans don't cover — accessible aging-in-place layouts, two-story stacks, or specific live-work configurations. Construction cost per square foot is roughly the same as a pre-approved plan at any given finish level; the premium is design plus time. About 30% of Boise lots cannot host any pre-approved plan without modification, which is the natural candidate pool for custom design.

Design review

Design review is the discretionary city review of an ADU's exterior design, scale, and materials. It is separate from plan check (which verifies code compliance). Pre-approved Boise plans skip design review entirely — that is what saves 4–8 weeks at the city. Custom plans trigger standard design review through the city's planning division. Lots inside the historic overlay (North End, Hyde Park, and a few pockets elsewhere) go before the Historic Preservation Commission. Lots in the Foothills hillside overlay (HS-O) face additional grading, landscape, and view-corridor review. HOA architectural committees layer their own review on top of city review and often add the most aggressive design constraints — Eagle subdivisions are a typical example.

Detached ADU

A detached ADU is a free-standing structure separate from the primary residence — its own foundation, its own walls, its own roof. Boise's six pre-approved plans (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey) are all detached units, and detached is the dominant ADU configuration in Boise post-2023 because the city zoning rewrite explicitly aimed to make backyard cottages easier to permit. Detached units rent at a small premium over attached units because tenants get full privacy with no shared walls. The trade-off is higher build cost: a new foundation, full exterior envelope, and an independent sewer lateral on most lots. Detached works on any lot that meets the 5,000 sq ft minimum and has buildable area after setbacks.

Easement

An easement is a recorded right granted to a third party — typically a utility company, neighboring property, or the city — to use a portion of the lot for a specific purpose. Common ADU-blocking easements include 10-foot rear utility easements (running power, gas, or telecom along the back fence), drainage easements that follow a swale through the lot, and access easements that protect a flag-lot driveway. Easements show up on the title commitment and on the recorded plat; they are not on the parcel-viewer map. Buildable area is calculated after subtracting all recorded easements, not just setbacks — which is why two adjacent lots of identical size can have very different ADU potential.

Garage conversion

A garage conversion turns an existing attached or detached garage into a permitted ADU. Cost typically runs $80,000–$150,000 (vs. $115,000–$345,000 for a detached new build), and the timeline is shorter because the foundation, walls, and roof already exist. Trade-offs: the dwelling is locked into the garage footprint (usually 400–600 sq ft), bringing slabs and exterior walls up to current Boise energy code can be expensive, and the homeowner loses covered parking. Some HOAs require a replacement garage if the original is converted, which can flip the math — at that point a new detached ADU on a different part of the lot often pencils better. Best fit: homeowners with no remaining yard for a detached unit.

Gross yield

Gross yield is annual gross rental income divided by total project cost, expressed as a percentage. It is the simplest income-property metric — no expenses, taxes, insurance, vacancy, or maintenance subtracted. A Boise Kingfisher rented at $1,650/month against a $200k all-in build returns roughly 9.9% gross yield. Gross yield is useful for fast screening across plans, neighborhoods, and rent-vs-buy scenarios; it is not a substitute for cap rate or cash-on-cash when underwriting a real deal. Operating costs on a Boise ADU typically run 25–35% of gross rent — property tax, insurance, vacancy reserve, repairs, and (in CC&R-restricted units) HOA fees — so net cap rate generally lands 2–3 percentage points below gross yield.

HELOC (Home Equity Line of Credit)

A HELOC is a revolving line of credit secured by the equity in a primary residence. Borrowers draw against the line during construction and make interest-only payments on the drawn balance during the draw period (typically 10 years), then amortize the remaining balance over a repayment period (typically 20 years). HELOCs are the most common ADU financing path in Boise because they preserve the homeowner's existing first-mortgage rate (no refi required) and the draw-as-you-build mechanic matches construction billing. As of May 2026, Idaho Central Credit Union (ICCU) advertised intro HELOC rates near 6.25% APR; Mountain America Credit Union (MACU) and most national lenders ran 1–2 points higher. Rates are variable and tied to prime.

Hillside overlay (HS-O)

The Hillside Overlay (HS-O) applies in Boise's Foothills neighborhoods. It triggers slope-tiered review: lots above certain grade thresholds face mandatory geotechnical engineering, foundation upgrades, grading limits, and view-corridor protections. Most Foothills ADUs end up custom because the pre-approved plans assume flat lots with standard 5–10 ft setbacks; foothills parcels often require engineered foundations, stepped construction, and restricted excavation footprints. Hillside review adds 6–12 weeks on top of standard permit review. Geotech reports alone typically cost $3,000–$8,000. Lots above the highest slope tier may not be buildable at all without a variance. Homeowners considering a Foothills ADU should pull the city's hillside-overlay map before purchase, not after.

Historic overlay

A historic overlay is a zoning layer that applies in Boise's designated historic districts — primarily the North End, Hyde Park, the East End, and a few smaller pockets. ADUs on overlay lots go before the Historic Preservation Commission (HPC) for additional design review focused on scale, massing, materials, and architectural compatibility with the historic primary structure. The overlay does not prohibit ADUs; it constrains how they look. Pre-approved plan exteriors sometimes need adjustment to clear HPC review, which can push a project back into custom-design territory. Timeline impact: 6–10 weeks of added review on top of standard plan check. Homeowners should pull both the city zoning map and the historic-district map before assuming a pre-approved plan will work.

HOA design review

HOA design review is the discretionary architectural review run by a homeowner association's architectural committee, separate from and on top of city design review. It is common in newer Boise, Meridian, and Eagle subdivisions and is often stricter than city standards — required matching siding, matching roof pitch, matching window grids, restricted colors, and (occasionally) outright bans on accessory structures. Approval timelines vary from 2 weeks to 3 months depending on how the committee meets. Boise homeowners should engage their HOA early; a quiet 'no' from the architectural chair after the city permit is already in hand is a worst-case timeline scenario. Approved drawings should be filed with both the city and the HOA before construction starts.

House hacking

House hacking is the practice of living in one unit of a property while renting out the other(s). On a Boise ADU lot, house hacking typically means living in the ADU and renting the larger primary house, or vice versa. Before December 2023, owner-occupancy rules required the property owner to live in one of the two units. The City of Boise removed that requirement under the Modern Zoning Code, so a Boise ADU lot can be operated entirely as a rental — both units leased, owner living elsewhere. House hackers most often choose owner-in-ADU because it minimizes their housing cost (small unit, lower utilities) while the larger primary captures the higher rent. The strategy is the fastest path to leveraged residential cash flow in the Treasure Valley.

HPC (Historic Preservation Commission)

The Historic Preservation Commission (HPC) is Boise's appointed design-review board for projects in the city's historic overlay districts — North End, Hyde Park, East End, and several smaller pockets. The HPC reviews exterior elevations, materials, scale, massing, window proportions, and overall compatibility with the historic primary structure before any overlay-district ADU can permit. Approval is discretionary, not ministerial: the commission can require redesigns. Hearings are public and run on a posted monthly schedule. Pre-approved plan exteriors sometimes need to be reworked to clear HPC, which can push a project back into custom design. Timeline impact: 6–10 weeks of additional review on top of standard plan check. Architects who routinely work North End ADUs are worth their fee in HPC navigation alone.

IRC (International Residential Code)

The International Residential Code (IRC) is the model code that governs construction of one- and two-family dwellings, including most ADUs. Published by the International Code Council, the IRC covers structural, mechanical, electrical, plumbing, energy, and life-safety provisions for residential structures up to three stories. Idaho adopts the IRC at the state level through the Idaho Building Code, and the City of Boise enforces the IRC with state amendments and Boise-specific energy and stretch provisions. Plan-checkers reference the current IRC edition when redlining drawings; inspectors reference it on site. Garage conversions and historic-building ADUs sometimes face the trickiest IRC compliance because existing assemblies have to be brought up to current standards — a frequent budget surprise.

Source: International Code Council (IRC)

Modern Zoning Code (MZC)

The Modern Zoning Code (MZC) is Boise's comprehensive zoning rewrite, adopted in June 2023 and effective December 1, 2023. It was the first ground-up rewrite of the city's zoning ordinance in roughly 60 years. The MZC raised the ADU size cap from 700 sq ft to 900 sq ft, removed the strict owner-occupancy requirement that had constrained ADU rentals, eliminated the extra off-street parking requirement, and opened ADUs in most residential zones (R-1A, R-1B, R-1C, R-2, R-3). The downstream effect was dramatic: Boise's monthly ADU permit rate jumped from ~3/month before the MZC to ~27/month after — a 9× increase measured 3.5 months in. The MZC is the controlling zoning framework for any current Boise ADU project.

Source: BoiseDev — Boise zoning rewrite passes (2023)

Owner-occupancy

Owner-occupancy is a zoning requirement that the property owner live in either the primary residence or the ADU. The intent is to keep ADUs from operating as detached rentals on absentee-owned lots. The City of Boise removed strict owner-occupancy under the December 2023 Modern Zoning Code; a Boise ADU lot can now be operated as a full rental with neither unit owner-occupied. Eagle currently retains an owner-occupancy provision, and several smaller Treasure Valley municipalities have versions of the rule. Idaho's SB 1354 (effective July 1, 2026) preempts city owner-occupancy rules statewide, so the remaining cities will have to drop the requirement at that date. Eagle homeowners planning ADU rentals should confirm the current local rule.

Pre-approved ADU plan

A pre-approved ADU plan is one that has cleared design review and standards review with the City of Boise in advance, so homeowners using it skip 4–8 weeks of city design review and avoid the $8,000–$15,000 typical architectural design fee. The City of Boise launched its pre-approved plan catalog on March 17, 2026, with 6 downloadable plans (Goldfinch, Waxwing, Kingfisher, Kestrel, Sandpiper, Osprey) plus 2 mirrored variants (Sandpiper B, Osprey B) marked Coming Soon. All eight plans were designed by Parke Architecture (Eamonn Parke, AIA). Material finishes can be customized freely; structural footprint, exterior elevations, and window placement cannot. Pre-approved plans cleanly fit roughly 70% of Boise lots.

Source: City of Boise — Pre-approved ADU plans

SB 1354

SB 1354 is Idaho Senate Bill 1354 — state legislation that preempts city owner-occupancy rules for accessory dwelling units, effective July 1, 2026. It forces remaining holdout municipalities (notably Eagle, which currently retains an owner-occupancy provision, and several smaller cities) to drop owner-occupancy requirements. The bill does not preempt other ADU rules — setbacks, size caps, design review, and permitting authority remain with each city. After SB 1354's effective date, every Idaho municipality that allows ADUs at all must allow them as standalone rentals without an owner-on-site. Boise already removed owner-occupancy in 2023, so SB 1354 has no in-city effect — but it removes a key Treasure Valley arbitrage where investor buyers had been steering toward Boise specifically.

Section 11-06

Section 11-06 is the portion of the Boise Municipal Code that contains residential use standards under the Modern Zoning Code. It is where the ADU-specific rules live — size cap, attached-vs-detached treatment, setback exceptions for accessory structures, height limits, and design standards. Planners and plan-checkers cite specific subsections of 11-06 in redline comments and permit conditions. Homeowners do not need to memorize the section, but they should expect to see it referenced on permit paperwork and should know that any builder or architect who claims to work Boise ADUs ought to be able to point to the relevant 11-06 subsections by heart. The current section text is hosted on American Legal Publishing under the city's BMC.

Source: Boise Municipal Code 11-06 (American Legal)

Setback

Setbacks are the minimum distances from each property line within which no permanent structure may be built. They are measured from the property line to the closest point of the structure (typically the exterior wall, sometimes the eave overhang). For accessory structures in most Boise residential zones, the side-yard setback is 5 ft and the rear-yard setback is 10 ft. Front-yard setbacks for accessory structures are usually deeper — the ADU cannot sit forward of the primary residence. Corner lots have two front yards and two side yards, which shrinks buildable area materially. Setbacks are the single biggest constraint on which pre-approved plan footprint fits a given lot, ahead of even lot size in many cases.

Site plan

A site plan is a scaled drawing of the lot showing existing structures, property lines, recorded easements, required setbacks, and the proposed ADU location. It is required for every ADU permit application in Boise and is typically the first document a plan-checker reviews — if the proposed footprint doesn't fit the buildable envelope on the site plan, nothing else matters. Site plans must show north arrow, scale, dimensions of every structure, distances to property lines, utility connections, parking spaces, driveway, and existing trees over a specified caliper. Site plans drawn by a licensed surveyor are cleaner than homeowner sketches and are required when easement or property-line ambiguity exists. Pre-approved plan applicants still need a lot-specific site plan even though the architectural drawings come from the city.

Speakable

Speakable is Schema.org structured-data markup that flags portions of a page suitable for voice assistants and AI search surfaces (ChatGPT search, Google AI Overviews, Perplexity) to read aloud or quote in a generated answer. Implementations use CSS selectors — typically h1 and a data-speakable attribute on the page's headline-answer paragraph. BoiseADU.com money pages emit speakable specifications in WebPage, Article, and Organization schema, pointing at the H1 and the headline-answer paragraph in each section. The goal is AI-citation surface: when an AI engine answers 'how much does a Boise ADU cost,' the speakable selector tells it which sentences to lift verbatim. This glossary itself is structured to be speakable per-term.

THOW (Tiny House on Wheels)

A Tiny House on Wheels (THOW) is a dwelling-style structure built on a trailer chassis rather than a fixed foundation. Under Idaho code, a THOW is classified as a recreational vehicle (RV), not as a permanent dwelling. That distinction is decisive in Boise: an RV cannot be permanently occupied on a residential lot, and a THOW cannot be permitted or built to IRC residential standards because it is not regulated under the IRC. Homeowners interested in the tiny-home aesthetic should look at the 280 sq ft Goldfinch pre-approved plan or a 396 sq ft Waxwing — both deliver tiny-home scale on a fixed foundation, fully permitted as legal ADUs with the rental, financing, and resale advantages a THOW cannot provide.

Variance

A variance is a discretionary deviation from a specific zoning standard — setback, height, lot coverage, parking — granted by the city when strict application of the rule would cause undue hardship not of the owner's own making. Variances are obtained through a formal application to the Planning & Zoning Commission with a public hearing; approval is uncertain and granted only on demonstrated hardship grounds (not 'I want a bigger ADU'). Most Boise ADU variance requests are denied because the hardship test is high; the practical path on a tight lot is usually a smaller plan, a different lot, or a custom design that works within the existing zoning envelope. Variance applications take 6–12 weeks and have non-refundable filing fees.

ZHVI (Zillow Home Value Index)

ZHVI is the Zillow Home Value Index — a smoothed, seasonally adjusted measure of typical home value for the 35th to 65th percentile of homes within a defined geography and housing type (single-family, condo, all-home). ZHVI is a market index, not a sale comparable: it tracks how the typical home value is trending across a city, ZIP code, or metro, but it does not value any individual property. ADU investors use ZHVI to model long-run appreciation contribution to total return (typical Boise ZHVI growth has averaged the mid-single-digits annually post-2020, with notable volatility). For appraisal — refinance, sale, insurance — use a licensed appraiser's report, not ZHVI.

Source: Zillow Research — ZHVI methodology

Zoning code

A zoning code is the body of municipal ordinances that governs how each parcel of land may be used, what may be built on it, and how big and tall structures may be. It includes zone designations (R-1A, R-1B, R-1C, R-2, R-3 in Boise's residential structure), use tables, dimensional standards (setbacks, height, lot coverage), and process rules for permits and variances. Boise's controlling document is the Modern Zoning Code (MZC), effective December 1, 2023, which replaced the prior code adopted in the 1960s. Neighboring jurisdictions — Meridian, Eagle, Nampa, Caldwell, Garden City, Star, Kuna, and Ada and Canyon counties — each maintain their own zoning codes, which is why ADU rules vary so meaningfully across the Treasure Valley.