ADU ROI in Boise: What Actually Drives the Return
Iron Crest does not publish market rent averages or projected ROI tables for Boise ADUs. This guide sets out what genuinely determines the return on an accessory dwelling unit — construction cost, achievable rent from local comparables, operating-cost drag and the property tax increase — and shows the recorded figures from one real project.
Quick answer
There is no percentage we can honestly publish. Iron Crest has no MLS access and no appraisal dataset, so any "adds a fixed percentage of your home value" figure on a contractor's website is a guess dressed up as data.
Most home improvement projects deliver one kind of return — whatever the work contributes to the property's value when you eventually sell. An accessory dwelling unit can do something a kitchen remodel cannot: it creates a separate, legally occupiable dwelling on land you already own, which means it can produce rent while you hold the property as well as adding appraisable square footage when you sell.
That structure is what makes ADUs interesting financially. It is also what makes them easy to oversell. The arithmetic only works if the inputs are real, and the two inputs that decide everything — what the unit actually costs to build on your lot, and what it will actually rent for in your neighborhood — are both property-specific. Neither can be read off a contractor's website.
This guide is written the other way round from most ADU ROI pages. Instead of printing rent averages and payback tables, it explains the four variables that decide the outcome, shows you where to get a defensible number for each one, and gives you the recorded figures from a single real Iron Crest project so you can see what one actual result looked like.
We do not publish market rent averages or projected ROI tables for Boise ADUs. Treasure Valley rents move enough that any figure we printed would be stale, and the honest source for a rent number on your property is current comparable listings from a property manager working your ZIP code. Iron Crest has no MLS access, no rent survey and no appraisal dataset — so a percentage, a cap rate or a payback period published here would be an invention, not a measurement. What we can show you is the recorded outcome of a real project we completed.
Recorded figures — North End detached ADU
- 600 sq ft, one bedroom, full kitchen and bath, covered porch
- Budget range: $120,000–$150,000
- Project duration: 16 weeks
- Appraised added value: $155,000
- Rental potential recorded for this unit: $1,400–$1,600 per month
These are this project's own recorded figures, for one 600 sq ft unit on one North End lot. They are not a Boise market average, not a rent quote for your property, and they do not predict what another lot will produce. Full budget, phasing and photos: 600 sq ft Detached ADU Build in North End Boise.
Four things decide whether an ADU works financially on a specific Boise property, and none of them is a market-wide average. Get a real number for each and you have a defensible projection. Skip any one of them and you have a sales pitch.
1. All-in construction cost
Not the square-foot price — the all-in number including design, engineering, City of Boise permit fees, trade permits, sewer and water connection work, and the site conditions nobody sees until excavation starts. Our Boise ADU work runs $95,000 to $320,000, averaging $165,000, and the spread inside that range is driven far more by utility connection distance, lot access and existing structure condition than by finish selections. Where to get your number: a line-item estimate on your actual lot, not a per-square-foot multiplier.
2. Achievable rent for that unit, in that neighborhood
Rent is set by the unit type, its finish level, whether it has in-unit laundry and a private entrance, and above all by the specific neighborhood. A one-bedroom detached unit in the North End and the same unit on a Bench lot are not the same product to a tenant. Where to get your number: a property manager who currently leases units in your ZIP code, working from live comparable listings rather than a published average. Ask for what units are actually leasing at, not what they are listed at.
3. Operating-cost drag
Gross rent is not the return. Subtract property management if you use it, a maintenance reserve for a standalone building with its own roof, envelope, plumbing and mechanical systems, the insurance increment for covering a second dwelling and its occupants, any utilities you carry rather than submeter, and realistic vacancy between tenants. Where to get your numbers: a management quote, an insurance quote for the specific configuration, and our ADU maintenance guide for what the unit will actually need each year.
4. The property tax increase
Ada County revalues the parcel to reflect the added improvement once the certificate of occupancy is issued. The increase depends on the assessed value placed on the unit and on the levy rates set annually by the taxing districts your parcel sits in — a contractor cannot forecast either. Where to get your number: the Ada County Assessor's office, for your specific parcel, before you commit to a budget.
Run the four in order and the arithmetic falls out on its own: monthly rent, minus operating costs, minus the monthly share of the tax increase, gives net monthly income. Multiply by twelve and divide into the all-in construction cost, and you have a payback period built on your own numbers rather than someone else's.
Construction cost is the one input on this page we can state directly, because it is ours. These are the ranges Iron Crest quotes for ADU work in the Boise area. They are ranges, not prices: a fixed number for your project comes from a site visit and a line-item estimate.
| ADU Type | Iron Crest Boise Range | What Moves the Number |
|---|---|---|
| Garage conversion to finished ADU | $95,000–$145,000 | Slab condition and slope, whether the existing walls and roof can be brought to dwelling standards, how far the sewer and water lines have to travel |
| New detached ADU (600 sq ft, 1 bed) | $145,000–$220,000 | Foundation type, equipment access to the rear yard, trench length for the sewer lateral and water line, panel capacity at the main house |
| Attached ADU addition | $120,000–$260,000 | How much of the existing structure has to be opened up, roof tie-in complexity, whether the main house electrical and plumbing can carry the added load |
| All Boise ADU work | $95,000–$320,000 (average $165,000) | The full range across every ADU type and size we build in the Boise area |
Building permit fees are calculated from the City of Boise fee schedule (Table 1-A, effective 10-1-21): valuations over $100,000 are charged $913.09 plus $5.17 per $1,000 over $100,000, plus 20% for residential plan review — about $1,220 on a $120,000 garage conversion and about $1,560 on a $175,000 one-bedroom. Trade permits for plumbing, electrical and mechanical are issued separately, and sewer and water connection fees are set by Boise Public Works. Full breakdown in our Boise ADU cost guide.
This is the sequence we walk homeowners through. It takes a few phone calls and produces a projection built entirely on numbers sourced to someone who is accountable for them.
Step 1 — Confirm what you can legally build
Before any number matters, confirm the unit is permittable on your lot. Boise allows an ADU of up to 900 sq ft or 70% of the primary structure, whichever is smaller, with a maximum of two bedrooms and two stories, and setbacks that follow the underlying zone (Zoning Code 11-03-03.2.B). Boise sets no maximum lot coverage percentage in its residential districts, so it is the setback envelope, not a coverage figure, that limits your footprint. Call Planning & Zoning at (208) 608-7100 with your parcel number.
Step 2 — Get an all-in construction number for your lot
A line-item estimate from a site visit, not a per-square-foot multiplier. It should name the foundation type, the trench route for sewer and water, the electrical service approach, the permit and plan-review fees, and a contingency. Utility connection distance is the single most common reason two visually identical ADUs cost very different amounts.
Step 3 — Get a rent comparable, in writing
Call two or three property managers who currently lease units in your ZIP code. Ask what comparable one-bedroom or studio units have actually leased at in the last ninety days — not asking prices, and not a metro-wide average. If two managers give you materially different numbers, ask each what comparables they used.
Step 4 — Price the operating costs
Management fee (or your own time), maintenance reserve, the insurance increment quoted by your agent for the actual configuration, any utilities you will carry, and a realistic vacancy allowance. Write each one down as a monthly figure.
Step 5 — Ask the Assessor about the tax impact
Ada County Assessor, with your parcel number and the proposed square footage. Ask how the improvement will be classified, how the homeowner's exemption applies, and which assessment cycle the change will land in.
Step 6 — Do the arithmetic yourself
Monthly rent, minus operating costs, minus the monthly share of the tax increase, equals net monthly income. Times twelve, divided into the all-in cost, equals payback in years. Then run it again with the rent ten percent lower and the cost ten percent higher, because that is the case you actually need to survive.
Regulatory stability is a real part of the long-term picture, and it is one of the few areas where the facts are published and checkable rather than estimated. Two sets of rules apply to a Boise ADU.
Boise's own ADU standards
Under Boise Zoning Code 11-03-03.2.B, an ADU may not exceed 900 square feet or 70 percent of the primary structure's floor area, whichever is smaller, with no more than two bedrooms and no more than two stories. Minimum lot area and maximum density standards do not apply to ADUs. Setbacks are the underlying zone's, and the design must be compatible with the principal structure. Boise's residential districts set no maximum lot coverage percentage, so the setback envelope and the size cap are what constrain the footprint. The Harris Ranch Specific Plan is the exception that still requires owner-occupancy and one parking space until Boise conforms to state law.
Idaho Code § 67-6541, as enacted
The statute takes effect July 1, 2026 and applies to cities over 10,000 in population, which must comply by February 1, 2027. It requires those cities to allow one internal or one detached ADU on a lot with a single-family dwelling, bars a size limit below 1,000 square feet or 75 percent of the primary dwelling, prohibits an owner-occupancy requirement, limits when parking may be required, caps fees at no more than those for a single-family dwelling, bars setbacks stricter than those applied to a single-family dwelling, and requires administrative approval as of right. Boise's current 900 sq ft / 70 percent cap therefore sits below the floor the statute sets — confirm which limit is in force for your permit with Planning & Development Services before you design to a square footage.
Why permitted status is the whole ballgame
An unpermitted unit is the one failure mode that destroys the return outright. An appraiser may decline to credit the square footage, a buyer's lender may refuse to count the rent, and retroactive permitting during a sale can mean opening finished walls to prove what is behind them. Every ADU Iron Crest builds is permitted through the City of Boise or the applicable Ada County jurisdiction, with the inspection record to show for it.
These are the recurring items that sit between gross rent and what you actually keep. We are not putting dollar figures on them, because every one of them is quoted to you individually and a made-up number here would only mislead your budget. Get a real quote for each before you commit.
Insurance
Either a dwelling coverage endorsement on your existing policy or a separate landlord policy. A detached structure and a basement conversion price differently, and a short-term rental needs its own endorsement or a commercial policy. Ask your agent to quote the specific configuration before construction starts, and get it in writing.
Property tax increase
Ada County revalues the parcel once the certificate of occupancy is issued. The amount depends on the assessed value placed on the improvement and the levy rates for your taxing districts. Ask the Ada County Assessor about your parcel; treat the answer as a fixed monthly line, not a rounding error.
Utilities
A separately metered unit lets the tenant pay their own water, sewer, power and gas, which is the cleanest arrangement. If the ADU shares the main house's meters, you carry the incremental cost and should build it into the rent. Veolia Water Idaho supplies Boise drinking water and the City of Boise handles sewer, so connection and billing arrangements come from two different providers.
Maintenance, vacancy and turnover
A standalone ADU has its own roof, envelope, plumbing, electrical and mechanical systems, all of which need servicing whether or not it is occupied. Add turnover cost between tenants and a vacancy allowance. Our ADU maintenance guide sets out what the unit will actually need through a Treasure Valley year.
Homeowners weighing more space often compare an ADU against a traditional addition. Both add square footage; they differ in what that square footage can do and in what it costs. The table below compares the two on factors we can state from our own work rather than from market data.
| Factor | ADU | Home Addition |
|---|---|---|
| Iron Crest Boise range | $95,000–$320,000 (avg $165,000) | $60,000–$400,000+ (avg $145,000) |
| Can it be rented separately? | Yes — it is a separate legal dwelling | No — it is part of the main house |
| Kitchen and bathroom required? | Yes — full kitchen, bath and private entrance | Only if the design calls for them |
| Permitting complexity | Higher — new dwelling unit, zoning and utility review | Lower — addition to an existing dwelling |
| Flexibility of use | Rent, family, guest suite or home office | Fixed household space |
| Tax treatment if rented | Rental income reporting and depreciation rules apply — ask your CPA | None — personal use |
The practical distinction is that an ADU can generate rent and an addition cannot, while an addition is simpler to permit and can start at a lower cost. Which is the better investment depends entirely on whether you will actually rent the unit, and at what rent — which brings you back to the comparables. Side-by-side detail: ADU vs. home addition in Boise. Tax treatment of rental property is a question for a CPA, not a contractor.
We do not publish nightly rates, occupancy assumptions or short-term rental revenue figures — we do not operate rentals, and there is no dataset behind such numbers. What follows is the structural difference between the two models, which is what you should be weighing before you go looking for real figures.
Long-term lease
One payment a month on a fixed term, minimal turnover cost, and very little of your time. The unit can be let unfurnished, so there is no furnishing budget and no cleaning cycle. Your exposure is concentrated in vacancy between tenants and in the quality of your screening. Idaho landlord-tenant law governs the habitability standard, notice requirements and the handling of security deposits, and those obligations apply whether or not your lease mentions them.
Short-term rental
A variable gross that depends on season, location and how actively the listing is managed, against a cost base a long-term lease does not carry: furnishing the unit, cleaning between every stay, platform commissions, higher utility consumption, faster wear on finishes, and insurance written for transient occupancy. It is a small hospitality business rather than a passive holding. The City of Boise regulates short-term rentals separately from long-term tenancies, and lodging taxes apply, so confirm the current registration and tax requirements with the City and the Idaho State Tax Commission before you plan around nightly rates.
How to decide: get a long-term rent comparable from a property manager and a short-term revenue estimate from an operator working your neighborhood, subtract the full cost base from each, and compare the net figures. Comparing gross rent against gross revenue will always flatter the short-term option, because that is the option with the costs hidden in it. If you intend to finish the unit for rental either way, our rental-ready ADU guide covers the finish selections that hold up between tenants.
How much does an ADU increase property value in Boise?
There is no percentage we can honestly publish. Iron Crest has no MLS access and no appraisal dataset, so any "adds a fixed percentage of your home value" figure on a contractor's website is a guess dressed up as data. What we can tell you is the recorded outcome of one real project: the 600 sq ft detached one-bedroom ADU we built in Boise's North End appraised at $155,000 in added property value, on a budget range of $120,000 to $150,000. That is that project's own figure, on that one lot, with that one main house — it is not a Boise average and it does not predict what your property will appraise at. What actually moves an appraisal is whether the unit is permitted and has its own entrance, kitchen and bathroom; how the added square footage compares to the main house; and what comparable properties with legal ADUs have recently sold for in your neighborhood. Unpermitted units are the clearest downside risk: an appraiser may give no credit for the square footage, and a buyer's lender may refuse to count it. For a real number, ask a local appraiser or agent to pull comparables on your specific parcel before you commit to a budget.
What is the typical payback period for an ADU in Boise?
We do not publish payback tables, because a payback period is only as good as the rent figure underneath it, and we have no rent survey to source one from. The calculation itself is simple enough to run yourself once you have real inputs. You need four numbers: the all-in construction cost including permits, utility connections and design (our Boise ADU work runs $95,000 to $320,000, averaging $165,000); the achievable monthly rent for that unit type in your specific neighborhood, which a property manager working your ZIP code can pull from current comparable listings; the operating-cost drag, meaning management fees if you use a manager, a maintenance reserve, the insurance increment and vacancy between tenants; and the property tax increase Ada County assesses once the certificate of occupancy is issued. Net rent after those items, divided into the construction cost, gives a payback period in years. Gross rent divided by cost is the number that makes ADUs look better than they are.
Is short-term rental or long-term rental better for a Boise ADU?
The honest answer is that it depends on inputs we cannot supply from a web page. We do not publish nightly rates, occupancy assumptions or short-term rental revenue figures for Boise, because we do not operate rentals and have no dataset behind such numbers. What we can set out is the structural difference. A long-term lease produces one predictable payment a month, minimal turnover cost and very little of your time. A short-term rental produces a variable gross that depends on season, location and how actively the listing is managed, and it carries costs a long-term lease does not: furnishing the unit, cleaning between every stay, platform commissions, higher utility use and wear, and an insurance product that actually covers transient occupancy. The City of Boise also regulates short-term rentals separately, so confirm the current registration, tax and occupancy requirements with the City before you build a financial plan around nightly rates. Before choosing, get a long-term rent comparable from a property manager and a short-term revenue estimate from an operator working your neighborhood, then compare net figures rather than gross.
How do property taxes change after building an ADU in Boise?
Building a permitted ADU will increase your Ada County property tax assessment, because the Assessor revalues the parcel to reflect the added square footage and improvements once the certificate of occupancy is issued. We do not publish an estimated dollar increase — the amount depends on the assessed value the Assessor places on the new unit and on the levy rates set each year by the taxing districts your parcel sits in, neither of which a contractor can forecast. Two points are worth raising with the Assessor's office before you build. First, ask how the added improvement will be classified, because whether any part of it falls under Idaho's homeowner's exemption depends on that classification and on how the unit is occupied. Second, ask when the reassessment will take effect, since it usually lands in the assessment cycle following occupancy rather than immediately. The Ada County Assessor can answer both for your specific parcel; treat the tax increase as a real line in your operating budget rather than a rounding error.
Does building an ADU affect my homeowner's insurance in Boise?
Yes. Adding an ADU changes what your policy has to cover, and most standard Idaho homeowner policies do not automatically insure a second dwelling unit or the people living in it. You will generally need either a dwelling coverage endorsement added to your existing policy or a separate landlord policy for the unit. We do not publish a premium figure, because pricing depends on the carrier, the replacement cost of the structure, your deductible and claims history, and whether the unit is occupied by family or rented. Liability is the part homeowners most often underestimate: if you rent the unit, a tenant injury or a tenant's damage claim needs coverage that a personal homeowner policy is not written to provide. If you intend to run a short-term rental, most carriers require a specific endorsement or a commercial policy, and operating without one can void coverage. Call your agent before construction starts, describe the unit and its intended use, and get the premium impact in writing so it goes into your operating budget rather than surprising you at renewal.
This guide is one piece of Iron Crest Remodel's ADU resource library. The guides below cover the inputs this page tells you to go and source.
The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.
Want Real Numbers for Your Boise ADU?
We will walk your property, confirm what is permittable on your lot, and give you a line-item construction estimate you can put into your own return calculation. Registered with the Idaho Division of Occupational and Professional Licensing and insured.