The Buyer's Math

Part 1 argued that California moved housing production onto individual homeowners. This part tests whether that actually works for someone trying to buy in and do it.

The honest answer is that it barely does — for reasons that have nothing to do with whether accessory dwelling units are a good idea.

I'm going to use a specific deal shape, because generalities hide the problem. A buyer purchases a Ventura County tract house with an attached garage. He converts the garage into an accessory dwelling unit (an ADU — a self-contained second residence on the same lot). He carves a junior accessory dwelling unit (a JADU — 500 square feet or less, created within the walls of the existing house) out of a downstairs bedroom. He adds a half bath to replace the bathroom the JADU takes with it, and builds a carport where the garage door used to be. He lives in the main house. Both new units rent long-term.

These are long-term tenancies of 30 days or more. Not short-term rentals, not vacation rentals, not transient occupancy, and not a rooming house. That distinction matters more than it sounds like it does, and I'll come back to it.

Four constraints that decide the deal before you look at a single listing

Short-term rental is off the table. ADUs and JADUs must be rented for terms longer than 30 days under state law. Senate Bill 543 tightened it further — JADUs are now barred from short-term rental outright. This isn't a local ordinance you can shop around; it's statute.

The Federal Housing Administration's 203(k) renovation loan can only build an attached ADU. The California Department of Housing and Community Development spells this out in its own ADU Handbook. If the garage is detached, the renovation loan can't fund the conversion. That's a hard filter on every listing you look at.

Only half the projected rent counts. For a proposed ADU under the Standard 203(k), lenders may count 50 percent of estimated rent toward qualifying, capped at 30 percent of total qualifying income. The 75 percent figure you'll read about applies to an ADU that already exists.

The county ceiling is real. Ventura County's Federal Housing Administration loan limit is roughly $1,035,000. Purchase plus rehabilitation has to fit underneath it at 96.5 percent loan-to-value, which caps total project cost near $1,072,000. Confirm the current figure with your lender before you make an offer.

Why conversion beats building above the garage

My first instinct was to build two units above a three-car garage. The arithmetic killed it.

Garage slabs in California tract housing are typically three and a half inches, unreinforced, with no vapor barrier, and they were never engineered to carry a second story. Underpinning or drilling piers runs $25,000 to $60,000 — and you discover it after you're in escrow, not before. New construction over a garage runs $400 to $500 per square foot in this county.

Two units above 600 square feet of garage, plus the JADU, plus soft costs and the contingency a 203(k) requires, came to roughly $600,000. Against a rehabilitation budget of about $255,000.

Conversion runs $150 to $220 per square foot. The same 600 square feet costs around $120,000 instead of $350,000, and the foundation problem disappears entirely because you aren't adding load anywhere new.

Three line items will still surprise the contractor's bid:

• A topping slab or framed floor over the pitched, unreinforced garage slab: $8,000 to $15,000
• Cutting the slab to trench kitchen and bath drain lines: $15,000 to $25,000
• Infilling the garage door opening with a header and shear wall, since the footing under a garage door opening usually isn't sized for a bearing wall: $6,000 to $12,000

State law helps here. When a garage is converted in conjunction with an ADU, the city cannot require replacement parking.

Build the carport anyway. It runs $15,000 to $30,000 on the existing driveway apron, and a four-bedroom tract house with no covered parking takes an appraisal adjustment and narrows your exit buyer pool. The family that wants four bedrooms wants a garage.

The buy box

• Attached garage, 400 square feet minimum, 600 preferred
• Four bedrooms, so the main house still functions as a three-bedroom after the JADU carve-out
• A downstairs bedroom on an exterior wall with an adjacent full bathroom
• Sewer, not septic
• No homeowners association, no coastal zone, no very high fire hazard severity zone
• $780,000 to $840,000, dated but structurally sound
• Camarillo, Simi Valley, Moorpark, Santa Paula, parts of Thousand Oaks

That third item eliminates most listings. In a lot of Ventura County tract plans the downstairs bedroom is interior, or the only downstairs bathroom is a powder room off the entry. No exterior wall means no independent entrance, which means no JADU — and a two-unit deal instead of a three-unit one.

The fourth item is why the buy box says four bedrooms and not three. A four-bedroom, three-bath becomes a workable three-bedroom, two-and-a-half-bath. A three-bedroom, two-bath becomes a two-bedroom, one-bath, and you've damaged both the main unit's rentability and its resale.

The numbers

$780,000 purchase plus $220,000 rehabilitation is $1,000,000 in total project cost. Down payment at 3.5 percent is roughly $35,000, plus closing costs.

| Monthly cost | Amount | |---|---| | Principal and interest | $6,150 | | Mortgage insurance premium | $450 | | Property taxes | $958 | | Insurance | $300 | | Total | $7,858 |

The ADU at $2,550 and the JADU at $1,800 produces $4,350 gross, or roughly $3,900 after vacancy and reserves.

Net housing cost for the main house: approximately $3,950 per month — about what a three-bedroom rents for in Camarillo.

That's the honest result. Not cash flow. Rental parity, while controlling a million-dollar asset on $35,000 down with tenants amortizing the loan. That is a genuinely good outcome. It is not the outcome anyone promises you.

Move out after the twelve-month owner-occupancy requirement and rent the main house at $4,200, and the property runs a few hundred dollars negative every month. As a pure acquisition play, this does not work.

Utilities: what you save and what it costs you

You can run all three units off the existing services rather than installing separate meters. State law helps — an ADU or JADU is not treated as a new residential use for connection fees or capacity charges, and a JADU cannot receive its own utility connection at all.

Separate water, electric, gas, and sewer connections for two units would run $40,000 to $95,000. Upsizing the shared services instead — a 200-amp panel upgrade, subpanels, a larger water service, water submeters where required — runs $18,000 to $35,000. Real money saved.

Two cautions. Allocate the shared bill by square footage or plumbing fixture count, never by number of occupants — a per-person utility charge financially penalizes households with children and creates familial status exposure under fair housing law. And understand that a mandatory utility charge is generally treated as rent, which means it counts against the cap under Assembly Bill 1482 where that law applies. You save at construction and absorb utility volatility inside a capped revenue line for the life of the hold.

The deduction you can't use yet

A cost segregation study on $220,000 of new construction is unusually clean — real invoices, fresh basis, no land component, and heavy 15-year land improvement content. Reclassify 25 to 30 percent and you're accelerating $55,000 to $65,000 into year one under permanent 100 percent bonus depreciation.

Then Internal Revenue Code Section 469 catches it. A long-term rental is passive by default. For a W-2 buyer, that deduction suspends and sits there.

It isn't lost — suspended losses release on a fully taxable sale of the activity — but it is not a shelter either, and California does not conform to bonus depreciation at all, so there's no state benefit and you carry a permanent basis difference.

The exception that would free it is the seven-day short-term rental rule, which takes the activity out of passive treatment entirely. And that's the first constraint on this page: closed by statute on these exact units.

So California deregulated the construction and simultaneously foreclosed the tax treatment that would make it pencil for the buyer the policy was aimed at. I've written more on which rental losses you can use and which you can't in the Next Level Real Estate Investing series.

A cost segregation study runs $3,000 to $6,000 on a project this size. Worth it when one of those doors is open. Pure expense when none are.

The cost nobody puts on the bid

Your ADU's certificate of occupancy can cost you the single-family exemption under Assembly Bill 1482 before a tenant ever moves in. That's Part 1 of The ADU Decision, and if you're doing this deal it's the most expensive page on this site.

And when you eventually sell, the home sale exclusion may not cover the whole property. Part 2 of that series covers why.

I am not a certified public accountant and this is not tax advice.

What this part proves

The buyer who needs this policy most — accessible financing, small down payment, first property — gets the version that barely works.

The deal is real. The arithmetic is thin. And every constraint that makes it thin was written into the same statutes that made the units possible.

Part 3 runs the identical construction for someone who already owns the land. Same units, same code, same county, completely different answer.