Capital Improvements That Reset Your Prop 13 Base Year (and the Ones That Don't): A 2026 Guide for Bay Area Luxury Owners

I had an Atherton client text me last week, mid-remodel, asking if the new pool he was putting in was going to send his property tax bill into a different stratosphere. I told him to call his CPA. Then I called his CPA myself, because I wanted to know what he was going to say.

I've been managing luxury rentals on the Peninsula for 12 years across Atherton, Palo Alto, Los Altos Hills, Woodside, Portola Valley, Hillsborough, and Menlo Park. About a quarter of my owner conversations this spring are some version of the same question: I'm thinking about a remodel, an addition, a pool, a wildfire hardening upgrade, or a backyard ADU. What's the assessor going to do to my property tax bill?

The answer is more nuanced than most owners realize. Some improvements absolutely trigger a reassessment of the new portion. Some don't. And a couple of categories California has specifically carved out as exempt — most owners don't know about.

Here's the actual playbook for 2026, with the caveat I tell every client: this is what I see happening in practice, but get a CPA and a tax attorney involved before you actually file anything.

How Prop 13 reassessments work in 2026

Quick recap. Proposition 13 (1978) caps property tax at 1% of assessed value, with annual increases limited to 2% or CPI, whichever is lower. That assessed value is your "base year value," typically set when you bought. For an Atherton owner who bought in 1998 at $4 million, the base year value today is $4 million plus 2% per year, which lands around $7.2 million. Property tax runs roughly $90,000 a year, not the $300,000-plus it would be at current market value.

That's the whole protection. The thing that erodes it is reassessment. Three triggers:

1. A change in ownership (the 1978 baseline rule) 2. New construction (the part this post is about) 3. A few narrow special cases (decline in value, calamity, transfer between spouses)

For luxury owners, change of ownership is usually well-understood. New construction is where the surprises hit.

The reassessment math

When you do new construction, the assessor doesn't reassess your whole property. They split it into two parts: the existing structure, which keeps your old base year value, and the new construction, which gets its own brand-new base year value at current market value.

So if your existing $7.2 million Atherton base gets a new $1.5 million ADU added, the ADU portion gets assessed at $1.5 million. Your blended assessed value goes from $7.2 million to $8.7 million. Your property tax bill goes up by about $18,000 a year. The original $7.2 million doesn't move.

Most people grasp that part. The interesting question is what counts as new construction.

What triggers a reassessment

Here's what I see actually trigger reassessment in San Mateo and Santa Clara counties on the high-end properties I work with:

Additions to the structure. New bedroom wing, expanded kitchen footprint, new garage, new ADU, new guest house. The new square footage gets reassessed at current market value.

New pools, spas, and major hardscape over a certain dollar threshold. A $250,000 pool with custom stonework and lighting will get the assessor's attention. A simple in-ground pool might not, depending on the assessor's discretion. SMC tends to be stricter on this than SCC.

Major mechanical upgrades that change substantial use. Wine cellar with climate control. Basement-to-media-room conversion. New detached studio.

Sport courts and large permanent outdoor structures. Tennis courts, outdoor kitchens, pool houses, pavilions.

Significant interior remodels that add functional value. This is the gray zone. A standard kitchen renovation usually doesn't trigger anything. But a "kitchen renovation" that doubled the kitchen footprint by absorbing the dining room, added a butler's pantry, and rerouted plumbing across the floor — that's getting flagged.

What does NOT trigger reassessment

This is the part most owners don't know, and it's saving people serious money in 2026:

Like-for-like replacements. Replacing a kitchen with a similar kitchen. Re-roofing. Repainting. Refinishing floors. New windows in existing openings. New HVAC equipment in an existing footprint.

Solar PV systems. Per the Active Solar Energy System exclusion in California Revenue and Taxation Code Section 73, solar PV installations don't trigger reassessment until the property changes ownership. When the next buyer takes over, the solar gets folded into the assessed value. The exclusion was extended in 2024 and remains in effect through 2026 and beyond.

Battery storage paired with solar. Same exclusion. Powerwalls and equivalent battery systems installed alongside PV are protected. A standalone battery with no PV is in a gray area, but the trend is toward exclusion.

Seismic retrofits. Earthquake reinforcement of an existing structure (foundation bolting, cripple wall bracing, soft-story retrofit) is excluded under Revenue and Taxation Code Section 74.5.

Wildfire hardening. Specific wildfire-hardening upgrades — Class A roof replacements, ember-resistant vents, defensible space hardscape — are excluded from reassessment when done on an existing structure. This was clarified in 2023 and is in active use as more Peninsula owners do hardening work.

Disability access modifications. Ramps, grab bars, widened doorways, accessible bathrooms — excluded under Revenue and Taxation Code Section 74.6.

Calamity reconstruction. If your property was damaged by a declared disaster and you rebuild, the reconstruction up to the original square footage and value is excluded.

ADU exemption (limited). Under SB 13 and follow-on legislation, an ADU built on a primary residence doesn't trigger a reassessment of the main house. The ADU itself gets a new base year value, but the main residence's existing base is protected. Important for owners with very low original bases.

Where this matters for spring 2026 remodels

The conversation I'm having most this season is about wildfire hardening combined with major remodel work. Owners on the Woodside and Portola Valley side are looking at the insurance situation and concluding they need to harden the structure aggressively. Some are bundling that with kitchen remodels, master suite expansions, or seismic work the home was due for anyway.

What I tell them: itemize the work in the contract. Have your GC break out the wildfire-hardening line items separately from the remodel line items. The hardening work is excluded. The remodel might or might not trigger something. Keep the documentation tight so when the assessor's appraiser comes by, you can show the breakdown.

The other big one is solar plus battery. A typical $80,000-$120,000 install on a 6,000-square-foot home in Atherton is exactly the kind of investment that, in a less-protected world, would generate a few thousand a year in additional property tax. Under the current exclusion it generates zero. That's free money, and a lot of owners don't realize the exclusion exists.

When the assessor actually shows up

The supplemental bill timing is the part most owners get wrong:

You finish construction. The city's building department signs off and sends a completion notice to the county assessor. The assessor opens a file (this can take 6 to 12 months, sometimes longer in SMC), sends an appraiser out for a desk review or a site visit, and assigns a new value to the new construction portion. They send a supplemental bill covering the period from the permit final to the next regular fiscal year.

So you do a remodel that finishes in March 2026. You probably hear nothing until late 2026 or early 2027. Then a supplemental bill arrives and a new annual bill kicks in starting July 1, 2027.

If you disagree with the assessor's valuation, you have a window to file an Assessment Appeal Application. In Santa Clara and San Mateo counties, the regular filing window is July 2 through September 15.

For owners who recently bought at peak prices and think the assessment overshot — particularly on property bought in late 2025 or early 2026 — Prop 8 (Decline in Value) appeals are the relevant lever. Both counties let you appeal when current market value has dropped below your assessed value. This is rare for Atherton given the Q1 2026 record sale comps, but it's not impossible if you bought a teardown in 2024 and the current land value has shifted meaningfully.

The piece luxury owners most often miss

When you sell, the buyer's purchase price becomes their new base year value. That's the change-of-ownership trigger. But the buyer also inherits whatever recent improvements you made — including the ones you got the wildfire hardening or solar exclusions for. Those exclusions only apply to the current owner. Once the property sells, they unwind, and the new buyer's assessment includes everything at current market value.

For owners thinking about selling in the next two to five years, this is worth modeling. A wildfire-hardening exclusion that's saving you $4,000 a year is also putting $50,000 of "future tax value" on the property when you sell. Not a reason not to do the work. A reason to think about timing.

What to do this month

If you're planning a remodel: - Get the Section 73 (solar), 74.5 (seismic), 74.6 (accessibility), and wildfire hardening exclusions into your contract scope - Have the GC itemize the line items in the bid - File the assessor's exclusion forms when the work is done; don't wait for them to come to you

If you've recently completed work: - Watch for the supplemental notice 6-12 months after final inspection - Cross-reference the new assessed value against what your CPA expected - The appeal window is July 2 through September 15 if you disagree

If you're considering buying: - Model the post-purchase property tax at current market value, not the seller's old base - Factor in any recent improvements that will be folded in

The Peninsula tax map is one of the most lopsided in the country — a property paying $30,000 a year next door to one paying $200,000, on an identical street. Prop 13 is why. Knowing what does and doesn't reset it is real money in 2026.

If you own a luxury property in Atherton, Palo Alto, Woodside, or Los Altos Hills and you're planning improvements this year, schedule a confidential consultation. I'll walk you through how the work will affect your rental positioning and what I've seen on similar properties this spring.

Sources

• Proposition 13 — California State Board of Equalization
• Revenue and Taxation Code Section 73 — Active Solar Energy System Exclusion — California Legislative Information
• Revenue and Taxation Code Section 74.5 — Seismic Retrofitting — California Legislative Information
• Santa Clara County Assessor — New Construction — Santa Clara County Assessor's Office
• San Mateo County Assessor — Property Tax Information — San Mateo County Assessor-County Clerk-Recorder
• California State Board of Equalization Property Tax Rules — California State Board of Equalization
• California Wildfire Hardening Property Tax Provisions — California State Board of Equalization