What Twin Falls Homeowners Need to Know About ADU Permitting Timelines This Spring

If you want to break ground on an ADU this summer, the honest answer is that your permitting window is already tight. A complete ADU project in Twin Falls, from first conversation to move-in, typically runs nine to eighteen months. The permitting portion alone can take two to four months, depending on how complete your application is and which agencies need to review it. Waiting until late spring to start feasibility work usually pushes construction into the following year.

We wrote this guide to give Twin Falls and Magic Valley homeowners a realistic map of the process, with links to the city, county, and state sources you can verify yourself. No fluff and no inflated numbers.

The Real ADU Timeline, Phase by Phase

Most homeowners underestimate how much work happens before a shovel hits the ground. Here is what each stage actually involves.

Feasibility and concept (4 to 8 weeks). Before you commission a design, you need to confirm your lot qualifies. That means checking zoning, setbacks, lot coverage limits, utility capacity, and any easements or recorded covenants. Applications that skip this step are the ones that fail their first plan review and lose months to corrections.

Design and permit preparation (6 to 10 weeks). Once feasibility is confirmed, a designer or architect produces the plan set: site plan, floor plans, electrical, plumbing, structural details, and energy compliance. The Twin Falls County Building Department currently reviews against the 2018 International Residential Code and International Building Code with Idaho Amendments, including a 30 psf snow load, 24-inch frost depth, and seismic design category C. Those numbers matter because your structural engineer has to design to them.

Plan review and permitting (6 to 12 weeks). In Twin Falls, your application typically goes through zoning review, structural review, electrical review, plumbing review, mechanical review, and, where applicable, fire marshal sign-off. If any single department flags an issue, the clock effectively restarts while you revise. The City of Twin Falls permit requirements page confirms that residential detached accessory buildings over 200 square feet require a permit, and lists the code exemptions that do and do not apply.

Construction (4 to 9 months). Garage conversions tend to finish in four to six months. New detached builds usually take six to nine. Weather, inspector scheduling, and material lead times all affect the final number.

Add those phases together, and the honest total, from first conversation to certificate of occupancy, is nine to eighteen months. If you are targeting occupancy by this fall, you need to start feasibility work in late winter. If you are targeting spring or summer of next year, starting now is the right move.

What Idaho House Bill 166 Actually Does (and Does Not Do)

There is a lot of confusion about this law, and several ADU sites across Idaho oversell what it accomplished. Here is what the statute actually says.

House Bill 166, passed in 2023, does two specific things. First, it prevents homeowners' associations from strictly prohibiting what the law calls "internal accessory dwelling units" on owner-occupied homesteads. Second, it voids any restrictive covenant entered into after July 1, 2023, that bans those units. You can read the final enacted text of HB 166 directly from the Idaho Legislature.

Three things the law does not do, despite what you may have read elsewhere:

1. It does not restrict cities or counties. The original version of HB 166 would have prevented local governments from banning ADUs, but that provision was removed in the Senate before passage. BoiseDev reported on the Senate amendments when they happened.
2. It only covers "internal" ADUs, meaning units located inside a detached, owner-occupied home or its attached or detached garage. Standalone backyard cottages on a new foundation are not covered by the statute.
3. It does not prevent HOAs from setting reasonable rules. Even where HB 166 applies, HOAs can still regulate size, height, setbacks, parking, and the number of bedrooms.

The practical takeaway: if your HOA has tried to block a garage conversion or an in-law suite inside your existing footprint, HB 166 may help. If you want to put a detached 900 square foot unit in your backyard, the law that governs your project is local Twin Falls zoning, not HB 166.

Why Local Rules Matter More Than State Law

Idaho's Local Land Use Planning Act is the statute that gives every Idaho city and county the authority to set their own zoning, setbacks, lot size minimums, parking requirements, and design standards. Two homeowners on lots twenty miles apart can face very different ADU rules.

For Twin Falls specifically, we recommend every homeowner do three things early.

Pull your parcel information from Twin Falls County GIS and confirm your zoning designation. Your zoning district determines whether an ADU is allowed by right, by conditional use, or not at all.

Call the Twin Falls Planning and Zoning office at (208) 735-7267 and ask specifically about ADU allowances in your zone, including minimum lot size, side and rear setbacks, maximum unit size, and any owner occupancy requirement.

Check your deed for any recorded covenants. HB 166 does not retroactively void covenants recorded before July 1, 2023, so older deed restrictions can still apply.

Doing those three things takes a few hours. Skipping them and finding out at plan review that your lot does not qualify can cost you months and thousands in wasted design work.

The Spring Timing Question

Permit departments do not publish monthly queue data, but in our experience working across Twin Falls and the surrounding Magic Valley, spring submissions tend to compete with winter backlogs that are still clearing. Summer submissions often move faster through review but land right as construction crews are booked out. Fall submissions usually see the cleanest review cycles, then move into winter planning and foundation work in early spring.

This is a pattern, not a rule. The more important factor is application completeness. A well-prepared submission in April will beat a rushed one in October almost every time.

A 30 Day Checklist for Homeowners Who Want to Build This Year

If your goal is to break ground in summer or early fall, here is what to do in the next month.

Verify your zoning and setbacks through Twin Falls Planning and Zoning. Get the answer in writing if possible.

Locate your water, sewer, septic, gas, and electric service points. Call your water provider to ask about the service capacity to your lot. If you are on septic, confirm your system can handle the additional load or whether a separate system will be required.

Pull your deed and any HOA covenants. Read them. Flag anything related to accessory structures, lot coverage, or occupancy.

Decide your ADU type. Internal conversion, attached addition, or detached new build. Each has a different cost, timeline, and permit implications.

Take dated photos of your lot from multiple angles, including the proposed build area, existing structures, utility connection points, and any slope or drainage features.

Start the financing conversation. Construction loans and HELOCs both require plan sets and appraisals, and some lenders take four to six weeks to close. Running financing in parallel with design, rather than after permits are issued, shortens your overall timeline significantly.

Financing and Material Lead Times

Two timeline factors that often get overlooked.

Financing moves on its own schedule. If your permit issues and your loan have not closed, construction does not start. Most local and regional lenders can fund an ADU construction loan in four to eight weeks, but that assumes clean underwriting and a complete plan set. Start the conversation early.

Material lead times have normalized compared to the supply chain disruptions of 2020 through 2022, but specialized items still run long. Electrical panels, certain HVAC equipment, and some plumbing fixtures can take four to eight weeks from order to delivery. Experienced contractors pre-order critical items as soon as permits are approved, not after the foundation is poured.

What an ADU Is Actually Worth

Numbers get thrown around a lot in this industry, so here is what the most defensible data actually says.

A 2025 Federal Housing Finance Agency analysis of California appraisal data from 2013 to 2023 found that properties with ADUs grew in median appraised value at an annualized rate of 9.34 percent, compared to 7.65 percent for properties without. That is a California-specific finding, and Twin Falls is not California. But the direction of the data is consistent across markets. Properties with income-producing secondary units tend to appreciate faster than comparable properties without them.

For Magic Valley rental income, current local market rents for a well-designed one-bedroom ADU typically fall in the $800 to $1,400 per month range, depending on location, finishes, and amenities. We recommend pulling current Zillow and Apartments.com comparables in your specific neighborhood rather than relying on regional averages.

Ready to Find Out What Your Lot Can Support?

If you have read this far, you are probably past the "is an ADU a good idea" question and into the "does my lot actually work" question. That is exactly what our free Readiness Call is designed for.

The call runs 10 to 15 minutes. We ask about your lot, your goals, and your timeline, then tell you honestly whether what you are picturing is realistic for your property. If it is, we'll walk you through the next step. If it is not, we tell you that too, and explain why. No pressure and no sales pitch. Just clear answers from a team that works with homeowners across Twin Falls, Jerome, Burley, Rupert, Filer, Buhl, Kimberly, Gooding, and Hagerman.

You can schedule your Readiness Call at twinfallsaduguys.com or call us directly at (208) 613-9830.

Common Questions

Does HB 166 mean I can ignore my HOA?

No. HB 166 only prevents HOAs from strictly prohibiting internal ADUs inside a home or its garage, and only under covenants created after July 1, 2023. HOAs can still regulate size, placement, parking, and use, and older covenants may still apply. Read your CC&Rs before you invest in design work.

What is the fastest way to get an ADU permit approved in Twin Falls?

Submit a complete, code-compliant application. Use plans prepared by a designer familiar with local codes. Respond to plan review comments within 48 hours. Most delays are self-inflicted through incomplete submissions or slow responses, not through the city backlog.

Does a garage conversion permit faster than a new detached unit?

Generally yes. Conversions reuse an existing foundation and building envelope, which reduces structural review scope and utility work. A typical garage conversion can take four to eight weeks, whereas a detached new build might take eight to twelve.

What happens if the city takes too long to respond to my application?

Under Idaho Code 67-6535, if you request reconsideration of a denial and the agency does not issue a written decision within 60 days, the request is deemed denied, and you may seek judicial review. There is no general statute that deems initial applications automatically approved because of delay, so practical follow-up with the department is what keeps your timeline moving.

Can I build an ADU on any lot in Twin Falls?

No. ADU eligibility depends on your specific zoning, lot size, setbacks, utility capacity, and any private covenants. Confirming those five things is the whole purpose of the feasibility phase, and it is the single most important step to complete before spending money on design.