What Building an ADU Does to Your Property Taxes in the Sacramento Region (2026)

Building an ADU does not reassess your whole property. Under Proposition 13, your existing house keeps its base year value exactly as it sits today. Only the new construction, the ADU itself, gets its own separate assessed value added on top. Two numbers, two different histories, one combined bill. That's the entire spine of this guide, and getting it right changes how you should actually think about the tax cost of building.

We're a contractor, not a tax advisor. Everything below explains how the mechanics work under state law. For what you'll specifically owe, that's a conversation with your county assessor's office or a CPA, not us. What we can tell you is what an ADU is likely to cost to build. Get that number first at SafewayQuickQuote.com, about two minutes, no site visit required, before you start running tax math on a project that hasn't been priced yet.

The One Rule That Fixes Most of the Confusion

Article XIII A of the California Constitution, the section written directly by Proposition 13 in 1978, sets the frame. Section 1 caps the ad valorem property tax rate at 1% of a property's full cash value, plus whatever voter-approved bonds and assessments layer on top in your specific tax rate area. Section 2 defines “full cash value” as the assessor's valuation “when purchased, newly constructed, or a change in ownership has occurred.” Outside of those three triggers, your assessed value can rise by no more than 2% a year under the inflation adjustment in subdivision (b), regardless of what the market does.

Building an ADU falls under “newly constructed.” Revenue and Taxation Code Section 75.10 spells out exactly what happens next: the assessor appraises the new construction at its full cash value as of the date it's completed, and that value becomes “the new base year value of the property or the new construction.” Read that phrase again. The statute says “or.” It's the new construction that gets a new base year value, not the whole parcel. Your existing home's base year value, the one anchored to whenever you bought the place or last triggered a reassessable event, doesn't move.

This is confirmed directly by the two definitions sitting side by side in the same chapter. Revenue and Taxation Code Section 70 defines “newly constructed” as an addition to real property, or an alteration amounting to a major rehabilitation or change in use. It's describing the new square footage, not the parcel as a whole. A homeowner in Elk Grove who adds a 600-square-foot detached ADU behind a house they bought fifteen years ago keeps that fifteen-year-old base year value on the house. The assessor adds a new, separate assessed value for the 600 square feet of ADU, calculated at what that construction is worth as of the day it's done. Two values, added together on the same bill, but only one of them is new.

Worth saying plainly: this rule protects the equity you've already built through Prop 13's 2% annual cap. A homeowner sitting on a 1998 base year value doesn't lose that protection by adding a backyard unit. The old value keeps compounding at its own slow, capped pace. Only the new square footage starts its own clock, on its own value, from the day it's finished.

How the Assessor Actually Values Your New ADU

Once the “only the new part” principle is clear, the next question is what number the assessor attaches to that new part.

Revenue and Taxation Code Section 75.10 sets the standard: the assessor values the new construction at its full cash value as of the date of completion. In practice, county assessors use the cost approach for this, estimating what it would cost to build the improvement new, then applying appropriate depreciation for anything short of brand-new condition, which for a just-finished ADU is generally close to none. That number tends to track fairly closely with what the project actually cost to build, adjusted to the assessor's own cost tables rather than your specific invoices.

Using the Sacramento region's established 2026 construction cost baseline as a planning reference, here's roughly what that means for the new assessed value, and the resulting annual tax add-on at a rate in the 1% to 1.2% range that's typical across this region's tax rate areas once voter-approved bonds and assessments are included:

Two things worth flagging about that table. First, these are planning ranges built from the region's construction cost data, not a promise about what any specific assessor's office will assign to your project; the assessor's own cost tables and your specific plans decide the real figure. Second, the rate itself isn't a flat, single number anywhere in this region. The constitutional 1% is a floor, not the whole rate. Voter-approved general obligation bonds for schools, community college districts, and local infrastructure measures get layered on top under Article XIII A, Section 1(b), and those vary by tax rate area, meaning your specific rate depends on exactly which city, school district, and special district boundaries your parcel sits inside. Your county assessor or tax collector's office is the only source that can tell you your parcel's actual combined rate.

One more detail that surprises people: the land itself generally isn't reassessed just because you built on part of it. Yolo County's own Assessor's Office states this directly in its published guidance on new construction: “The land value will not usually be changed due to new construction of a structure.” The new value attaches to the improvement, the ADU, not the dirt underneath it.

The Supplemental Assessment: The Bill Nobody Expects

Here's where most of the genuine confusion actually lives, and it has nothing to do with the size of the number. It's about timing, and it catches homeowners off guard because it arrives as a completely separate bill from the one they're used to.

California's regular property tax roll only updates once a year. But new construction doesn't wait politely for the next lien date to get finished, so state law created a second mechanism: the supplemental assessment, governed by Revenue and Taxation Code Sections 75.10 through 75.16. It exists specifically to catch the gap between when your ADU is done and when the regular annual roll catches up to reflect it.

Revenue and Taxation Code Section 75.11 lays out exactly how this works, and it splits on when the ADU is completed:

Completed June 1 through December 31. You get one supplemental assessment. It's the difference between the new base year value (your ADU's value) and whatever taxable value was already on the current roll for that portion of the property, prorated for the number of months remaining in the fiscal year, which runs through June 30.

Completed January 1 through May 31. State law calls for two separate supplemental assessments under Section 75.11(a): one covering the tail end of the current fiscal year, and a second covering the value change as it carries onto the roll already being prepared for the next fiscal year.

Yolo County's own Assessor's Office publishes a plain-language example of the proration math, built for a change-of-ownership scenario but running on the identical mechanic: a property purchased in September for $150,000 more than its prior assessed value generates a supplemental assessment on that $50,000 difference, prorated for the months remaining in the fiscal year, at 1% of the increase per the constitutional cap, before any local bonds or assessments are added.

Applying that same math to new construction: say a detached ADU in this region is completed on October 1, adding roughly $250,000 in new assessed value. The fiscal year runs July 1 through June 30, so nine months remain after an October 1 completion. At a combined rate of roughly 1.1%, prorated for those nine months, that works out to somewhere around $2,000 for the supplemental bill alone, covering that partial first year. Starting the following fiscal year, the full $250,000 in added value shows up on the regular annual bill at the full 12-month rate, before Prop 13's 2% annual cap starts applying to that new base value each year after.

That supplemental bill doesn't quietly show up folded into your existing mortgage escrow or your usual November and April installments. Before it arrives, county assessors mail a Notice of Supplemental Assessment (the state-standard form is BOE-67-B) showing the new value. The actual supplemental tax bill follows separately, and it's genuinely easy to miss if you're not expecting a second bill in the mail months after your final inspection.

What Does NOT Trigger a Reassessment

This is the useful side of the same coin, and it's where a lot of remodeling content gets vague. Revenue and Taxation Code Section 70 draws a real line between new construction and ordinary upkeep, and Yolo County's own published guidance states the practical version of that line about as plainly as a government document gets:

“To maintain your property's current value, you might need to paint your home or make some repairs. Typically, normal maintenance (such as painting or a new roof) would not cause a reappraisal and there would be no new assessment added by the Assessor.”

That single sentence covers more ground than most homeowners assume. Re-roofing an existing house: not new construction. Repainting inside or out: not new construction. Replacing a water heater, a furnace, or other worn mechanical equipment on a like-for-like basis: normal maintenance, not new construction. A kitchen refresh that swaps cabinets, countertops, and appliances without expanding the footprint generally falls in the same bucket, since it's restoring the property rather than adding new value beyond its prior condition.

The line moves once work crosses into “major rehabilitation.” Section 70(b) defines that as any rehabilitation, renovation, or modernization that converts an improvement to “the substantial equivalent of a new improvement.” A full gut-to-the-studs remodel that effectively rebuilds a structure, or a conversion that changes the property's use entirely, like turning an uninsulated garage shell into a permitted, code-compliant living space, does cross into new construction, and does get its own new assessed value added. That's exactly the category an ADU garage conversion sits in: not maintenance, a genuine change of use with real new construction value attached, even though the exterior walls might not move an inch.

Two narrower carve-outs worth knowing exist too, layered directly into the Constitution itself. Article XIII A, Section 2(c) lets the Legislature exclude active solar energy systems, fire sprinkler and detection systems, and accessibility modifications for disabled residents from the definition of “newly constructed,” and the Legislature has done exactly that for solar in particular. None of those exclusions apply to a standard ADU addition, but they're useful to know if your project also includes solar or an accessibility retrofit alongside the ADU itself.

The Rental Income Side of the Math

None of this tax math means much in isolation. The reason most Sacramento-region homeowners build an ADU anyway is the income side of the ledger, and it's worth putting the two numbers next to each other rather than treating the tax bill as a standalone cost.

Take the detached ADU example from above: roughly $1,800 to $4,100 a year in added property tax once it's on the regular roll. Compare that against a single month of rent on a comparable unit. Our Davis ADU rental income guide walks through realistic 2026 rent ranges by unit size for this region in detail, but the general shape holds across most Sacramento-region cities: a two-bedroom detached ADU commonly rents for well more than the entire year's added property tax in a single month. Even a smaller garage conversion, sitting at the low end of the tax table above, usually clears its own added tax cost in well under a month of rent.

That's not a promise about your specific numbers, your specific market rent, or your specific tax rate area, and we're a construction company saying it, not a financial advisor. It's a reason to run the actual math for your address rather than assume the tax line item alone should decide whether the project pencils out.

Timing: When the Assessment Actually Lands

Revenue and Taxation Code Section 75.12 defines “completion” for these purposes, and it's not automatically the same date as your final building inspection, even though the two usually land close together in practice. Completion is generally the earliest of: the date the ADU is available for use by the owner, the date it's actually occupied with the owner's consent if that comes sooner, or, for property that can't functionally be used yet on the “available” date, the date it can actually be used or occupied given the type of property involved.

That completion date is what starts the clock on your supplemental assessment described above. It's also worth knowing that if your ADU is only partly finished as of the January 1 lien date, the assessor can still place a partial value on the unfinished construction as of that date under related rules in this same statutory chapter; a project that drags across a January 1 lien date doesn't automatically escape assessment just because the certificate of occupancy hasn't been issued yet.

Practically, that means the sequence looks like this for most Sacramento-region ADU projects: permit issued, construction happens, final inspection clears close to your actual completion date, the county's supplemental assessment process picks up that completion date sometime after, a Notice of Supplemental Assessment arrives in the mail, and a separate supplemental tax bill follows. Then, starting with the next full fiscal year, the ADU's assessed value is baked permanently into your regular annual property tax bill, subject from that point on to the same 2% annual inflation cap that protects the rest of your assessed value under Prop 13.

County Assessor Contacts for the Sacramento Region

Every dollar figure and every deadline in this guide is general. Your parcel's actual assessed value, actual tax rate area, and actual supplemental assessment timeline live with your specific county assessor's office. Here's where to call, verified directly against each office's own published contact information:

Call before you assume anything about your specific bill. Assessor staff can look up your parcel's actual tax rate area, walk you through what a comparable ADU has been assessed at recently, and tell you exactly how the supplemental process will play out for your completion date. None of that is something a contractor, including us, is positioned to tell you with certainty. We build the ADU. The assessor sets the value. A CPA or tax professional is the right call for anything involving your personal tax planning or liability.

What We Verified, and Where We Drew the Line

Every statutory citation in this guide was read directly at leginfo.legislature.ca.gov: California Constitution Article XIII A, Sections 1 and 2, for the 1% rate cap, the 2% annual inflation limit, and the “newly constructed” trigger; Revenue and Taxation Code Section 70 for the definition of new construction versus normal maintenance; Section 75.10 for the “new base year value” standard applied to new construction specifically, separate from the whole parcel; Section 75.11 for the exact supplemental assessment mechanics and the January-through-May versus June-through-December split; Section 75.12 for how “completion” is defined; and Section 75.14 confirming this entire supplemental chapter applies specifically to property subject to Article XIII A. The Yolo County Assessor's own published pages on New Construction and Supplement Assessments, read directly, supplied the maintenance-versus-new-construction language and the proration example we adapted for the ADU scenario above. All four county assessor addresses and phone numbers were verified directly against each county's own current website.

What we deliberately left out: any promise about what your specific ADU will be assessed at, since that depends on your county assessor's own cost tables and your actual plans. Any single, fixed combined tax rate for the region, since voter-approved bonds and assessments vary by tax rate area down to the parcel level. Any claim about a specific dollar exemption or credit tied to your homeowner's exemption interacting with a supplemental bill, since that's a filing-specific calculation best confirmed directly with your assessor. And anything resembling personal tax advice. We're a licensed general contractor, not a CPA, and we're not going to pretend otherwise.

Frequently Asked Questions

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This article explains general property tax mechanics under California law. It is not tax advice. Confirm your specific assessed value, tax rate, and filing deadlines with your county assessor's office or a licensed tax professional.

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• Sacramento ADU Guide: Permits, Costs and EZPermit (2026)
• Davis ADU Guide: Permits, Costs and the City's Free Pre-Approved Plans (2026)
• Davis ADU Rental Income 2026: Will It Actually Pay for Itself?
• How Sacramento-Region Homeowners Actually Finance an ADU in 2026
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