ADUs and SB 9 Lot Splits: The Tax Questions Nobody Asks Until Afterwards
by Stephen Morris CPA, MBT, CCIM
Adding an ADU or splitting a lot under SB 9 affects three separate tax systems:
- Property tax
- Annual income tax
- Tax on the eventual sale
Most owners receive advice about permits and planning requirements, but little or no guidance about the tax consequences.
The order in which you take each step matters, and some decisions cannot easily be reversed.
Property Tax: Only the New Part Is Assessed
First, the good news.
Building an ADU does not generally trigger reassessment of your entire property. The county assesses the value of the new construction and adds it to your existing assessed value.
Your main house keeps its Proposition 13 base value, while the ADU receives a new assessed value.
You should budget for the resulting increase, but do not assume that your entire property tax assessment will reset.
Income Tax: A Rented ADU Is a Small Rental Business
Once you rent the ADU, you generally have:
- Schedule E rental income
- Depreciation on the new structure
- Deductible operating expenses
The ADU is treated as a separate asset with its own depreciation schedule.
Because it is new construction, qualifying shorter-life components may also be eligible for bonus depreciation.
Keep complete records of the construction costs. These records establish your tax basis, and reconstructing them several years later can be difficult and time-consuming.
The Sale: Where the Surprises Appear
This is the part many owners fail to model in advance.
If you sell a home with a rented ADU, the primary-residence exclusion may apply to the portion used as your home, but not necessarily to the rental portion.
Any depreciation claimed on the ADU may also be subject to recapture.
If you split a lot under SB 9 and sell the new parcel, two major tax questions arise.
The first is how to allocate your original tax basis between the retained property and the new parcel.
The second is whether the IRS views you as an investor selling property or as a dealer selling inventory.
Dealer treatment can result in:
- Ordinary income tax rates
- No access to a 1031 exchange
- Different reporting obligations
A single sale of one subdivided lot may still be treated as an investment transaction. However, a repeated pattern of splitting and selling property can begin to look like a development or dealing business.
SB 9 Has Its Own Conditions
An SB 9 lot split may come with specific conditions, including an owner-occupancy commitment of three years.
The law has also been subject to ongoing legal challenges and interpretation since it was introduced.
Obtain current land-use advice before proceeding, and establish the tax structure before recording the lot split or making binding commitments.
Will Adding an ADU Reassess My Entire Property?
No.
Only the value of the new construction is generally assessed. Your existing Proposition 13 base remains in place.
Can I Complete a 1031 Exchange on a Lot I Split Off?
Potentially, yes, if the parcel is genuinely held for investment.
If it is part of your home’s garden or backyard and is sold as a quick flip, the position becomes more complicated.
Seek advice before the split or sale.
I Rent My ADU and Live in the Main House. What Changes When I Sell?
The primary-residence exclusion may apply to the portion used as your home.
The ADU portion may be treated as rental property, including potential depreciation recapture.
The allocation between the two parts is worth calculating before the property is listed for sale.
Does the ADU Need Its Own Cost Segregation Study?
Not always.
Detailed construction records may provide enough information to identify and separate qualifying shorter-life components.
This is best addressed during the construction process rather than after the project has been completed.
Building an ADU or Splitting a Lot?
Talk to us before you record anything.
Tell us what you own, and where you're headed.
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