Yes, an ADU increases your property taxes. But in California it increases them far less than most homeowners fear, because of how Proposition 13 treats new construction. Your house is not reassessed. Only the ADU is.
Here is how the assessment works, what the first tax bill after construction looks like, and how a garage conversion or junior ADU is treated.
Will Building an ADU Trigger a Full Reassessment?
No. Under Proposition 13, your property’s assessed value is based on its purchase price and rises by no more than 2 percent a year. Two events reset that: a change in ownership, and new construction. When you build, only the new construction is assessed at its current market value. The existing house and land keep their assessed value.
In practice, your new assessed value is:
your existing assessed value + the assessed value of the ADU
If you bought your home many years ago and your assessed value is far below market, that gap survives the ADU. It is the reason an ADU on a long-held Peninsula property adds relatively little to the tax bill compared with what the same unit would cost a new buyer in taxes.
How the ADU Is Valued
The county assessor values the new construction, not your project budget. The figure is typically based on the cost of construction and the market value the new improvement adds, and it can come in above or below what you actually spent. Once set, that value becomes the ADU’s base year value, and like the rest of your assessment it can rise by no more than 2 percent a year after that.
The tax itself is the assessed value multiplied by your area’s tax rate. In California that rate starts at the 1 percent base levy set by Proposition 13, plus voter-approved bonds and local assessments. The total varies by tax rate area, so the most reliable way to estimate is to look at the combined rate on your current bill and apply it to an estimate of the ADU’s value.
The Supplemental Tax Bill
The first sign of the ADU on your taxes is usually a supplemental tax bill, not your regular annual bill. When new construction is completed, the assessor issues a supplemental assessment for the difference between the old and new values, prorated for the part of the fiscal year remaining after completion.
It catches people off guard because it arrives separately from the annual bill and is usually not paid through a mortgage impound account. Set money aside for it when the ADU is finished. From the following year on, the ADU’s value is included in your regular annual bill.
Garage Conversions, JADUs and Additions
Not every ADU is a new building, and the assessment follows what was actually built.
| Project | What gets assessed |
|---|---|
| New detached ADU | The new building, at its market value |
| Attached ADU | The added square footage and any new construction within the house |
| Garage conversion to an ADU | The value added by the conversion. Converting a structure to a different use counts as new construction, but the garage shell already on the roll is not assessed again. |
| Junior ADU inside the house | Only the value of the alteration work, which is usually modest |
| ADU over a garage | The new second story and any structural work |
Because a conversion reuses an existing shell, it typically adds less to the assessment than a new detached unit of the same size. That is one more reason the garage conversion and over-the-garage options are worth comparing early.
What Doesn’t Change
- Your base year on the main house. It stays where it is.
- The homeowners’ exemption. If you live in the main house and claim it, you keep it.
- Your assessment if the ADU is never permitted. An unpermitted unit may not be on the tax roll, but that is not a saving. If it is discovered, the assessor can add it to the roll retroactively through an escape assessment, and an unpermitted unit carries far bigger costs than tax. See what happens if you build without a permit.
Does the Tax Increase Outweigh the Value?
For most owners who rent the unit, no. The added tax is a fraction of what the ADU can earn, and it is a predictable cost you can plan for from the start. For owners who build an ADU for family, the tax is a cost of the use rather than an investment question. We cover the other side of the equation in how much value an ADU adds.
Your tax position is one input to the decision. The bigger ones are what the lot allows and what the unit will cost to build, and those are settled in the drawings. Renovation Drafting & Design prepares permit-ready ADU plans across Monterey, Carmel, Pacific Grove, Pebble Beach and Carmel Valley. See our ADU design service or contact us.
This article is general information, not tax advice. For an estimate specific to your property, contact the Monterey County Assessor’s office or a tax professional.
ADU Property Tax FAQs
Does an ADU increase property taxes in California?
Yes, but only by the assessed value of the ADU. Under Proposition 13, new construction is assessed at market value and added to your existing assessment. Your main house and land keep their existing assessed value.
Will building an ADU reassess my whole house?
No. Only the new construction is reassessed. The existing home keeps its Proposition 13 base year value, which continues to rise by no more than 2 percent a year.
What is a supplemental tax bill for an ADU?
When an ADU is completed, the county assessor issues a supplemental assessment for the added value, prorated for the rest of the fiscal year. It arrives as a separate bill, usually not covered by a mortgage impound account. After that, the ADU is included in your regular annual bill.
Does a garage conversion ADU raise property taxes?
Yes, by the value the conversion adds. Converting a garage to a dwelling counts as new construction, but because the garage structure is already on the tax roll, a conversion typically adds less to the assessment than a new detached ADU.
Does a junior ADU increase property taxes?
Only by the value of the alteration work, since a JADU is created within the existing house. The increase is usually small.
Do I lose my homeowners’ exemption if I rent out an ADU?
No. If you continue to live in the main house as your principal residence, you keep the homeowners’ exemption on it.