Adding an ADU can instantly raise a California property's value by 20% to 35%, and that one-time boost then compounds with the market for as long as you own the home. This calculator projects your property's value over 30 years — with and without an ADU — so you can see how much of the long-term payoff comes from the unit itself versus ordinary appreciation.
In this guide:
- How to Use This Calculator
- Understanding Your Results
- Where the Growth-Rate Presets Come From
- The 25% Instant Value Boost
- Assumptions & Limitations
How to Use This Calculator
Only two inputs drive the projection, and results update as you type:
| Field | What to Enter |
|---|---|
| Current Property Market Value | What your home would sell for today — use a recent appraisal or an estimate from comparable sales in your neighborhood. Note this is the market value, not the assessed value on your tax bill (which Prop 13 usually keeps far lower). |
| Annual Property Value Growth Rate | Pick one of the three historical presets (explained below) or choose Custom Rate to model your own scenario, from 0% to 25% per year. |
Understanding Your Results
The Equity Growth Analysis cards show snapshots at 5, 10, 20, and 30 years. Each card compares the projected property value Without ADU (your current value compounding at the chosen rate) against the value With ADU (the same compounding applied to your value plus the 25% boost), and the Difference — the equity attributable to the ADU at that point in time.
The chart plots both trajectories over 30 years. Notice that the gap between the two lines widens every year: because growth is compounding, the ADU's one-time boost also grows at the market rate. A $150,000 initial difference can more than triple over a 30-year hold at historical growth rates.
The collapsible Year-by-Year Equity Details table lists both the values and the difference for each year and can be downloaded as a CSV.
Where the Growth-Rate Presets Come From
The three presets are average annual appreciation rates for California home values over different look-back windows, so you can choose how much history you want your projection to lean on:
| Preset | Basis | Best For |
|---|---|---|
| 5.8% | 20-year average (2000–2020) — a full market cycle including both the mid-2000s boom and the 2008–2012 crash. | Conservative long-horizon planning is the default because it averages through a severe downturn. |
| 7.2% | 10-year average (2010–2020) — the post-recession recovery decade. | A middle scenario that reflects the modern supply-constrained California market. |
| 9.6% | 2024's year-over-year average appreciation. | An optimistic scenario; single hot years rarely persist for 30 years. |
Real markets don't move in straight lines — any single year can swing far above or below these averages. The presets are best read as "if the future resembles this slice of the past, on average," which is also why comparing two presets side by side is often more informative than any single projection.
The 25% Instant Value Boost
Appraisal studies and resale data in California consistently find that adding an ADU raises a property's value by roughly 20% to 35%, driven by the added living space, rental income potential, and multigenerational flexibility. The calculator uses a deliberately conservative 25% — near the middle-low end of that range — applied once, at completion.
Your actual uplift depends on the ADU's size and quality relative to the main home, local rental demand, and how appraisers in your area treat ADUs. A detached 1,000 sq. ft. unit in a high-rent coastal city will typically add proportionally more value than a small garage conversion in a lower-rent area. If you have a local appraisal or comparable sales, the Custom Rate option won't change the boost — but you can double-check the "Difference" figures against what ADU-equipped homes actually sell for nearby.
Assumptions & Limitations
The projection applies a single constant growth rate to both scenarios, assumes the 25% boost occurs immediately upon completion, and ignores construction costs, financing, selling costs, and taxes. It measures property value, not net profit — to weigh the value gain against what you'd spend building, start with the Construction Cost Calculator and the Break-Even Calculator.
Also, remember that market value and assessed value move very differently in California: Prop 13 caps assessed-value growth at 2% per year regardless of the market. Our Property Tax Reassessment Calculator covers that side of the equation.
