ADU Glossary

ADU (Accessory Dwelling Unit)

A secondary, self-contained living unit on the same lot as a primary single-family home, with its own kitchen, bathroom, sleeping area, and entrance. Also called: granny flat, in-law suite, backyard cottage, casita, secondary suite.

In Twin Falls, ADUs are defined and regulated under Title 10, Section 10-6-10-B of the Uniform Development Code. State law (Idaho HB 166 and SB 1354) protects the right to build ADUs in most residential zones.

Related: JADUdetached ADUattached ADUgarage conversion

JADU (Junior Accessory Dwelling Unit)

A smaller ADU built within the walls of an existing primary residence (typically a converted bedroom or interior space), usually capped at 500 sq ft. JADUs share some systems (entry, kitchen) with the main home and are common in California; Idaho code does not separately define JADUs, so all Twin Falls ADUs follow standard ADU rules.

Detached ADU

A standalone ADU built as a separate structure on the property, physically detached from the primary residence. Most common type in Twin Falls — backyard cottage, granny flat, or stand-alone unit.

Detached ADUs offer maximum privacy + design flexibility but cost more than garage conversions due to fresh foundation, framing, and exterior costs.

Attached ADU

An ADU built as an addition to, or carved out of, the primary residence — sharing at least one wall with the main home. Typically converts existing structure (basement, attic, addition wing) into a self-contained second unit.

Garage Conversion ADU

An ADU created by converting an existing attached or detached garage into livable space. Typically the cheapest ADU type ($80,000–$150,000 in Twin Falls) because foundation and shell already exist.

Related: detached ADU

Setback

The minimum required distance from a property line that a structure can be built. Twin Falls setbacks vary by zoning district and structure type — for example, R-2 requires 30' front, 7' side, 30' rear for the primary residence.

ADU setback rules in Twin Falls follow Title 10-6-10-A. Buildings 15 ft tall or under typically need 20' front, 3' side, 3' rear; taller buildings have larger side and rear setbacks varying by district.

Lot Coverage

The percentage of a residential lot covered by structures (primary residence, ADU, garage, etc.) — typically capped at 30–45% by zoning district. Determines how much building footprint your lot can support.

The 45% Rule

Twin Falls Title 10-6-10-B caps ADU size at 45% of the principal residence's living space. A 2,000 sq ft home can support a maximum 900 sq ft ADU. Calculated on living space only (excludes garage, unfinished basements).

Egress

A safe path out of a dwelling in case of emergency. Code requires every habitable room (especially bedrooms) to have egress — typically a window of minimum dimensions or a direct exterior door, sized so a person can escape and emergency responders can enter.

Impact Fee

A one-time fee paid to local government to fund infrastructure (water, sewer, roads, parks) needed to support new development. Idaho impact fees on ADUs typically run $6,000–$10,000, separate from the building permit fee.

Zoning Use Permit

A Twin Falls-specific permit ($250 fee) required for ADU construction, distinct from the building permit. Issued by Twin Falls Planning & Zoning after a 7-step review with a 15-day appeal window.

Building Permit

The primary construction permit required for any structural work on an ADU. Fee varies by project valuation. Issued by the local building department after plan review and required prior to foundation work.

Trade Permits

Separate permits required for electrical, plumbing, and mechanical (HVAC) work on an ADU. Filed alongside the building permit. Each has its own inspection schedule.

Certificate of Occupancy (C of O)

The final document issued after all inspections pass, certifying that the ADU is safe and legal to occupy. Required before a homeowner can move in or rent out the unit. Without a C of O, lenders may refuse to finance, insurers may deny claims, and resale becomes complicated.

Soft Costs

Project costs that aren't physical materials or labor — design fees, permits, impact fees, soil testing, surveying, financing fees, insurance. Typically 15–25% of total ADU project cost.

Hard Costs

Physical construction costs — foundation, framing, plumbing, electrical, HVAC, finishes, appliances. Typically 75–85% of total ADU project cost.

HELOC (Home Equity Line of Credit)

A revolving credit line secured by your existing home equity, drawn against as needed during construction. Common ADU financing path in Idaho for homeowners with significant equity. Typical 2026 rates: 7.5–10% variable.

Related: construction loancash-out refinance

Construction Loan

A short-term loan specifically for construction projects. Funds released in stages (draws) tied to inspection milestones. Often converts to a permanent mortgage at Certificate of Occupancy. Typical 2026 rates: 8.5–11% during construction.

Cash-Out Refinance

Replacing your existing mortgage with a new larger mortgage and taking the difference in cash to fund the ADU. Best when your current mortgage rate is high enough that giving it up makes sense.

FHA 203(k) Loan

A federally backed renovation and construction loan with lower down payment requirements (3.5%). Strict contractor requirements; HUD-approved consultant required for projects over $35,000.

Mini-Split (Ductless HVAC)

A heating + cooling system with an outdoor compressor connected to one or more indoor units. Common in ADUs because no ductwork is needed, installation is cheap, and the system is highly efficient.

Stub-Out

A capped utility connection point (water line, sewer line, electrical conduit) installed during early construction and left ready for future hookups. ADUs often share stub-outs from the primary residence rather than installing separate service.

Footing / Foundation

The structural base that supports the ADU. In Twin Falls, foundations must extend below the frost line (32–36 inches typical) to prevent damage from soil freezing and thawing. Foundation work runs $12,000–$25,000 for a detached ADU.

Frost Line

The maximum depth at which groundwater freezes in winter. Foundations must extend below the frost line to prevent heave. Magic Valley frost line: typically 30–36 inches depending on local soil and elevation.

R-Value

A measure of insulation effectiveness — higher R-value means more insulation. Idaho ADU walls typically require R-21 in framed walls and R-49 in attics to meet current energy code; Magic Valley winters demand the higher end of these ranges.

Idaho House Bill 166 (HB 166)

Idaho state statute (2023, Idaho Code 55-3212 / 55-618). A homeowners-association law, NOT a zoning law: it stops an HOA or private deed restriction from banning an 'internal' ADU (a unit built inside an owner-occupied home or its garage) on a detached, owner-occupied homestead. It does NOT bind cities or counties and does NOT grant a statewide right to build an ADU — the city-ban-preemption was removed in the Idaho Senate. The law that requires cities to allow ADUs is SB 1354.

Related: sb-1354

Idaho Senate Bill 1354 (SB 1354)

Idaho state statute signed March 31, 2026, effective July 1, 2026, codified at Idaho Code §67-6541. The law that actually requires Idaho cities to allow ADUs — but only cities OVER 10,000 population (locally: Twin Falls, Jerome, Burley). Bound cities must allow at least one ADU per single-family lot by-right and update their codes by February 1, 2027, and cannot require off-street parking, charge higher fees than for a single-family home, cap ADU size below 1,000 sq ft (or 75% of the primary dwelling), or require owner-occupancy. Cities under 10,000 are not bound.

Related: hb-166

Twin Falls Title 10-6-10-B

The section of the Twin Falls Uniform Development Code that governs ADUs. Establishes the 45% size rule, design consistency requirements, utility sharing requirement, parking requirement, and lists the stated purposes of the ADU ordinance.

Design Review

A jurisdictional process that evaluates the visual + architectural design of a proposed structure against local standards. In Twin Falls, ADU design review focuses on matching the primary dwelling's roof pitch, siding, and windows. In Wood River Valley cities (Ketchum, Hailey, Bellevue), design review is more comprehensive.

Pre-Fab / Modular ADU

An ADU manufactured in a factory and delivered to the site as a complete or partial structure. Typically 8–14 weeks on-site after a 2–4 month manufacturing lead time. Base prices look lower than custom but all-in installed costs (foundation, delivery, utilities, permits) usually narrow the gap.

Site-Built (Custom) ADU

An ADU built entirely on the property from foundation up using conventional construction methods. Slower than prefab but offers unlimited design flexibility and easier code-compliance with Twin Falls design consistency requirements.

Multigenerational Housing

A living arrangement where two or more adult generations of a family share a single property. ADUs are commonly used to support multigenerational arrangements — aging parents, adult children, or live-in caregivers in a separate dwelling unit on the family property.

Owner-Occupancy Requirement

A rule requiring the property owner to live in either the primary residence or the ADU as their primary residence. Some Idaho cities have historically imposed this; SB 1354 (2026) limits how strictly cities can require this for ADUs.

Feasibility Check

A professional on-site evaluation of a property to determine ADU viability before investing in design. Includes measurements, zoning verification, utility capacity, layout placement, and a budget range. Twin Falls ADU Guys offers a Feasibility Check for $250–$750, credited toward construction.