In Los Gatos, an ADU's Value Isn't Set by What It Costs to Build

Ask three contractors what a backyard ADU costs in Los Gatos and you'll get three defensible numbers, anywhere from $100,000 for a garage conversion to $520,000 or more for a detached unit on a hillside lot. Ask a lender what that same finished unit is worth once it's standing, and the conversation changes entirely. Permits, comps, and paperwork start to matter more than square footage or finish level.

That gap, between what an ADU costs and what a lender or appraiser will actually recognize, is the number most cost guides skip. It's also the one that determines whether building one is a smart move or an expensive mistake for anyone thinking about a Los Gatos property as an investment, not just a place to live.

Los Gatos is a market where this math matters more than most. The median home sold near $2.4 million over the three months ending in May 2026, and homes were moving in around 14 days on average. Land is scarce and demand hasn't slowed. That combination is exactly why ADUs keep coming up in conversations with buyers who see a lot and start doing math about what else could fit on it.

The gap between build cost and recognized value

Here's the part that catches people off guard. Appraisers and lenders generally require permits before they'll treat an addition, remodel, or ADU as contributory to a home's value. A unit built without final permits and inspections is often discounted in an appraisal, sometimes excluded from the value calculation altogether, regardless of the quality of materials or how much the owner spent building it.

A backyard cottage is worth what a lender will recognize, not what the contractor charged to build it.

This is why the most reliable way to estimate what an ADU actually adds to a property is to look at recent local sales of comparable homes with permitted ADUs against similar homes without one. Cost guides can tell you what construction runs. Only comps and permit records tell you what a bank will lend against.

If you're evaluating a property with an existing unpermitted structure in the backyard, the math gets more complicated, not less. You may be buying square footage that a future lender won't count.

What lot you're on already changes the budget

Before permits and appraisals even enter the picture, location on the map does a lot of the work in setting your starting cost. Los Gatos runs from the flat valley floor near Highway 17 up into the Santa Cruz Mountains, and that geography shows up directly in ADU pricing.

The hillside premium isn't a rounding error. Sloped lots typically require a soils report and an engineered foundation once the grade exceeds 10 percent, and that alone can add $15,000 to $40,000 before retaining walls or drainage work are factored in. Across the board, Los Gatos ADU costs tend to run 15 to 25 percent above the broader Bay Area average, driven by hillside engineering, the town's design review expectations, and general construction costs in a high-value market.

If you're comparing two properties, one flat and close to downtown, one on a slope in the hills, the ADU line item in your budget should look nothing alike, even before you get to what a lender will eventually recognize.

The tree ordinance that quietly adds weeks

Los Gatos protects specific trees, and if one sits where you're planning to put a foundation, it becomes part of your timeline whether you planned for it or not.

A permit is required to remove or significantly prune certain trees. On most developed residential lots, that threshold is a trunk 12 inches or more in diameter. In designated hillside areas, the threshold drops to 8 inches for specific species. Any oak, California buckeye, or Pacific madrone reaches "Large Protected Tree" status at 24 inches, and any tree of any species at 48 inches or more qualifies regardless.

A few practical details worth knowing before you site an ADU:

• The permit fee is $250 for the first tree and $125 for each additional tree in the same application.
• The Town Arborist generally decides within 10 to 15 business days of a complete application.
• Removal typically requires planting two or more replacement trees, and the permit is only valid for 90 days, so timing matters.
• If your ADU footprint affects a protected tree, expect to budget for an arborist report and a root protection plan as part of your development application, not as an afterthought.

None of this is disqualifying. It's a scheduling and budgeting variable that a lot of buyers don't discover until they're already deep into design.

The reassessment isn't what most people assume

A common assumption is that building an ADU triggers a full reassessment of the entire property's tax bill. That's not how it works. Santa Clara County generally reassesses only the value the new construction adds, not the whole parcel. Your existing assessed value stays where it was under Proposition 13 protections, and the new square footage gets its own added assessment.

That distinction matters when you're running numbers on whether an ADU pencils out as a long-term hold. The tax increase is tied to what you built, not to what your home happens to be worth on the open market the year you build it.

Financing shapes what you can actually absorb

Most Bay Area ADU projects get financed through a HELOC, a cash-out refinance, or a dedicated ADU construction loan, and the mix leans heavily toward home equity products. Across a set of more than 100 Bay Area ADU projects tracked by one regional builder, HELOCs accounted for roughly 48 percent of financing choices, cash-out refinancing for about 32 percent, and specialized ADU loans for the remaining 20 percent.

The equity math behind those choices is worth understanding before you start pricing contractors. Most lenders want 15 to 20 percent equity to remain in the primary residence after ADU financing closes, which caps combined loan-to-value around 80 to 85 percent of appraised value. On a $2 million Los Gatos home, that generally translates to roughly $1.6 to $1.7 million in total financing capacity across the first mortgage and any new ADU debt combined. That's the ceiling the appraisal gap runs into directly. If the appraiser won't count the ADU as contributory value because it lacks final permits, your available equity for the next project shrinks with it.

What changed on January 1

If you're planning any remodel or addition alongside an ADU, there's a fresh regulatory layer worth knowing about. The Town Council adopted Ordinance 2380 on December 2, 2025, bringing Los Gatos into compliance with the 2025 California Building Standards Code effective January 1, 2026. Along with the base code adoption, the Town added local reach code requirements: an AC-to-heat-pump replacement requirement that applies when an air conditioning unit is proposed to be replaced in an existing single-family home, and electric-readiness pre-wiring requirements for remodels or additions where gas or propane equipment is being installed. The town's Code of Ordinances is now codified through Ordinance No. 2382, enacted February 17, 2026, reflecting this change as the current standard.

For anyone converting a garage or adding an ADU that touches existing HVAC or gas lines, this is a real line item to build into the budget conversation with your contractor now, not something to discover mid-permit.

Before you underwrite the numbers

A short sequence worth working through before you commit to a build or price it into an offer:

1. Confirm zoning, overlay status, and any hillside or historic restrictions with the Planning Division before pricing anything.
2. Get the actual site assessed for slope and soil conditions. Flatland and hillside budgets aren't close.
3. Check whether protected trees sit inside the planned footprint and build the arborist review timeline into your schedule.
4. Pull the permit and final inspection history on any existing structure before assuming it adds appraised value.
5. Ask your lender directly how they treat combined loan-to-value and what documentation they require before an ADU counts toward equity.
6. Model the tax impact as an addition to your current bill, not a full reassessment of the property.

This is the kind of sequencing our team leans on collaboration with Renovato Builders to work through with clients directly, because the construction side and the transaction side of an ADU decision are the same decision, not two separate ones.

FAQ

Does building an ADU trigger a full property tax reassessment in Los Gatos? No. Santa Clara County generally reassesses only the value the new construction adds, not the parcel's full assessed value.

Will an unpermitted ADU count toward my home's appraised value? Usually not fully. Lenders and appraisers typically require permits and final inspections before treating a unit as contributory to value, and unpermitted space is often discounted or excluded.

Do I need a permit to remove a tree to make room for an ADU? If it's a protected species, oak, California buckeye, or Pacific madrone at 24 inches or more in diameter, or 8 inches in designated hillside areas, yes. The Town Arborist typically decides within 10 to 15 business days, and removal usually requires planting replacement trees.

What changed with Los Gatos building codes at the start of 2026? The Town adopted the 2025 California Building Standards Code effective January 1, 2026, along with reach code requirements that apply heat-pump and electric-readiness standards to certain HVAC replacements and additions involving gas equipment.

If you're weighing an ADU against a lot you already own or a property you're considering, the numbers above are the starting point, not the finish line. Every parcel in Los Gatos carries its own slope, tree canopy, and permit history, and that's exactly the kind of detail worth walking through with someone who can read both the construction side and the transaction side at once. Renovato & Co. is happy to look at your specific situation together. Let's Begin.