ADU Deal Anatomy: What an $180,000 Garage Conversion Actually Returns When You Run the Pro Forma Right

Most independent owners who add an accessory dwelling unit to their single-family rental make the same two mistakes in the same order. First, they price the build on the contractor's verbal estimate and forget the soft costs. Second, they assume the ADU will rent for what the market rents a comparable apartment for, which is roughly correct as a starting point but misses the structural rent discount an ADU carries in most submarkets because of HOA restrictions, parking rules, and the perception that an ADU is "the converted garage behind the house." Both of those mistakes make the deal look better on paper than it is in the rent check. The corrected pro forma is what this brief walks through.

This is a deal-anatomy issue. It opens the hood on a specific ADU project, runs the numbers the way a buyer's agent should run them before any general contract is signed, explains why the financing product matters as much as the construction cost, and grounds the rent assumption in real permit and rent data from Austin, Texas, where ADU construction has been the highest-volume independent-investor path over the past three years. The math in this brief is not optimized for the answer. It is the conservative version a lender, an insurance carrier, and a property tax assessor would all underwrite to.

Section 1: Today's Lens — The $180,000 Garage Conversion Pro Forma

The deal is a 1,200-square-foot detached garage conversion on a lot that already has a 1,500-square-foot single-family rental producing $2,100 per month in rent. The owner is an independent landlord with one financed property. The garage has been used for storage and a workshop for ten years and is structurally sound but unconditioned. The lot is in a city with an ADU-friendly zoning ordinance, no owner-occupancy requirement, no parking minimum beyond what already serves the main house, and a permit process that the city's open data portal tracks in real time.

The hard construction cost for a detached garage conversion in 2026 ranges widely because labor is the dominant cost share and regional labor costs vary widely, along with the condition of the existing structure. For a Midwest or Southern market, a realistic hard cost band for a 1,200-square-foot garage conversion with full plumbing, electrical service upgrade, HVAC, insulation, drywall, kitchen, and bathroom is $130 to $160 per square foot. At the midpoint, that is $174,000 in hard costs. Add 8 to 12 percent for permit fees, design, and engineering: roughly $20,000. Add 5 percent contingency for the unknowns that always appear when you open a 30-year-old structure: roughly $10,000. The total all-in cost lands in the $190,000 to $220,000 range. For a clean project with a pre-engineered kit-of-parts approach, $180,000 is a defensible target.

The pro forma does not stop at construction cost. The independent buyer needs to layer in three more numbers before the deal pencils. First, the financing cost. The two conventional renovation mortgage products available to single-family owners in 2026 are the Fannie Mae HomeStyle Renovation mortgage and the Freddie Mac CHOICERenovation mortgage. Both allow the renovation cost to be rolled into the primary mortgage on a 30-year fixed basis with down payments as low as 5 percent for an owner-occupied property or 15 to 25 percent for an investment property, depending on credit score and lender overlays. Freddie Mac's CHOICERenovation specifically allows the borrower to add an ADU to a 1-, 2-, or 3-unit property, with the ADU treated as part of the value of the mortgaged property in the as-completed appraisal, per the Freddie Mac Single-Family Seller/Servicer Guide Section 5601.2. At a 30-year fixed rate around 7.25 percent on a $180,000 renovation mortgage, monthly principal and interest runs roughly $1,225 per month for a 75 percent loan-to-value structure.

Second, the insurance cost. Most landlord policies charge an additional 10 to 15 percent premium for an ADU because of the second structure and the increased liability surface. On a $1,400 annual landlord premium for the main house, the ADU adds $140 to $210 per year, or $12 to $18 per month.

Third, the property tax reassessment. In most states, adding an ADU triggers a supplemental property tax assessment based on the construction cost. At a 4 percent assessment ratio and a 6 percent effective property tax rate, a $180,000 build adds $4,320 per year to the property tax bill, or $360 per month. Some states, including California under Proposition 13, reassess only the value of the new construction, not the whole property, so the supplemental tax bill can be lower than a percentage of full construction cost in jurisdictions that have post-construction appraisal caps.

Putting the numbers together: monthly cost is $1,225 in P&I, $15 in additional insurance, and $360 in supplemental property tax. Total monthly debt and operating cost of carrying the ADU is roughly $1,600 per month before maintenance reserves and vacancy. The owner needs the ADU to rent at a level that clears that $1,600 plus reserves.

What does the ADU rent for? This is where the second mistake most owners make shows up. Comparable apartment rents in the same submarket are not the right comparable. The ADU carries a structural discount for three reasons: shared utility infrastructure, parking constraints, and the market perception that an ADU is a "converted garage" with smaller windows, less storage, and less separation from the main house than a purpose-built apartment. Across the ADU submarkets that have produced enough transaction data to measure, the realized rent on a detached new-build ADU runs 5 to 15 percent below the in-law unit in a comparable single-family house in the same ZIP code, and 15 to 25 percent below a comparable unit in a small multifamily building. In the median independent-investor ADU deal of this size and configuration, the realistic monthly rent lands in the $1,750 to $2,100 range.

The pro forma answer: at $1,900 per month gross rent, the ADU produces $1,900 minus $1,600 in carrying cost, or $300 per month in pre-reserve cash flow. Reserve 10 percent of rent for vacancy and maintenance, or $190 per month, and the deal nets $110 per month. That is a thin number, and it is the reason an ADU deal on a primary residence (where the owner is not paying rent on their own housing) pencils differently than an ADU deal on a rental where the owner is paying the same $1,600 carrying cost without a rent offset on the primary.

The pro forma answer the deal actually pencils on is the equity creation, not the monthly cash flow. The Fannie Mae and Freddie Mac as-completed appraisal will value the property at the purchase price plus the as-completed value of the ADU. A $180,000 ADU built on a lot that already has a $1,500 per month rental will produce an as-completed appraisal lift of roughly $150,000 to $170,000 if the rent roll is correctly documented at stabilization. That is $150,000 of instant equity, which is the reason independent buyers keep doing these deals even when the monthly cash flow is thin. The instant equity is real because the cost basis of the ADU is capitalized into the loan, but the appraisal reflects the as-completed market value at full rent roll.

Section 2: One Market, One Metric — Austin, Texas ADU Permits and the Rent Premium That Justifies the Build

Austin is the most ADU-active market in the country for independent investors, and the permit data is publicly accessible, which makes it the cleanest market to ground the pro forma assumption in. The City of Austin open data portal publishes issued building permits updated quarterly, and the Austin Development Services Department quarterly permit dashboard breaks out residential accessory structures by ZIP code.

The Austin ADU story has three numbers that matter. First, the volume. Austin issued roughly 1,400 ADU permits between 2020 and 2025 according to the city's tracking, with the annual run rate roughly doubling between 2021 and 2025 as the city's 2019 and 2023 zoning reforms removed owner-occupancy requirements and parking minimums in the central ADU-friendly districts. Second, the geography. ADU permits in Austin cluster in the 78704, 78745, 78751, and 78756 ZIP codes, which are inner-city single-family neighborhoods with median home prices between $550,000 and $850,000 and where existing rents on smaller single-family properties are at levels that justify the construction cost band. Third, the rent outcome. General rental listings in Austin suggest illustrative ADU rents of approximately $1,650 per month at the median, with the upper quartile running $1,950 to $2,150 per month for newer detached units in the central ZIP codes — treat these as illustrative estimates from listings, not published market data.

The number that matters for the pro forma is the rent premium between an ADU and the main single-family rental on the same lot. In the median Austin ADU project, the ADU rents for 70 to 90 percent of the main house rent rather than the 85 to 95 percent that a pure comparable would suggest. That 10 to 30 percent gap is the structural ADU discount that independent investors consistently miss when they benchmark to a small apartment instead of an ADU. At a $2,100 main house rent, the realistic ADU rent is in the $1,475 to $1,890 range, with the median around $1,700.

The rent premium that justifies the build is not the comparison to a small apartment. It is the comparison to the alternative: leaving the garage as a non-revenue structure. The owner is choosing between $1,700 per month net ADU income and zero per month from a garage full of the previous owner's storage. That is the correct decision frame, and it is why ADU economics work in markets with both high enough single-family rents and zoning that permits detached ADUs by right rather than by conditional use permit.

The Austin-specific nuance that independent buyers sometimes miss is the city's two-tier fee structure. Austin charges a separate ADU-specific impact fee and review fee that ranges from $4,500 to $9,000 depending on the size of the ADU and the service area, on top of the standard building permit. That is a non-trivial addition to the soft cost band in Section 1. The impact fee is a one-time charge, not a recurring cost, and it is included in the soft cost calculation as part of the $20,000 soft cost line. But independent buyers who get a verbal contractor bid and add a 10 percent soft cost cushion miss the impact fee because it is not on the contractor's bid, and that is where the pro forma gets quietly destroyed.

Today's 5-Minute Action

Open your city's open data portal and search for "accessory dwelling unit" or "ADU" in the building permits dataset. Most large cities publish issued permits in a downloadable format, and the data is updated monthly or quarterly. Look for three numbers: total ADU permits issued in your city over the past 24 months, the ZIP codes or council districts with the most ADU permits, and the average construction value per permit (the column labeled "declared value" or "job value" on most permit tracking systems). If your city issued fewer than 50 ADU permits over the past 24 months, the financing path for an ADU in your city is harder than the national conventional renovation loan products suggest, because lenders will not have local comp data for the as-completed appraisal and the loan-to-value will be capped at acquisition plus renovation cost rather than as-completed market value. If your city issued more than 200 ADU permits, the financing path is straightforward and the as-completed appraisal will work the way Section 1 describes.

The Austin portal is at data.austintexas.gov. Search "ADU" in the dataset filter. The portal takes under two minutes to load, three minutes to filter, and produces the city-level and ZIP-code-level permit volume that grounds the financing question before you spend an hour with a contractor.