A backyard home, done right.
Real 2026 ADU costs, the actual LA County permit path for Altadena, and CSLB-verified builders — no sales theater, just the numbers you need before you break ground.
Know what you\u2019re building
Four ways to add a home to your lot.
Detached ADU
A standalone backyard home — full kitchen, bath, and its own address in the yard.
$250K–$450K+Garage Conversion
Turn the garage you don’t use into a legal rental or family suite.
$120K–$250KJunior ADU (JADU)
Up to 500 sq ft inside or attached to your existing house. The lightest path.
$60K–$120KCasita / Guest House
A flexible backyard room for family, work, or income — built to outlast trends.
$180K–$380KRanges reflect typical 2026 LA-area projects. Your lot decides the real number — that\u2019s what the quiz is for.
Where the money goes
Five things that actually move an ADU budget.
Site access & slope
Cranes, tight side yards, and Altadena’s hillside lots move real numbers, not rounding errors.
Utilities & sewer
New sewer laterals, panel upgrades, and trenching across a mature yard add up fastest.
Size & bathrooms
Each bathroom and every added 100 sq ft carries disproportionate cost per foot.
Finish level
Builder-grade to high-end is routinely a 2× swing on the same footprint.
Permit path
Unincorporated LA County has its own checklist — setback, fire, and grading rules apply.
The Altadena path
Permits here run through LA County — not a city.
Feasibility
Lot lines, setbacks, and what your parcel actually allows — before you spend on plans.
Design & engineering
Plans sized for LA County review, including fire and hillside overlays where they apply.
County submittal
Altadena permits through LA County Public Works — no city hall. Right checklist matters.
Build & final
Inspections in sequence, then the certificate of occupancy that makes it a legal ADU.
We explain the process — we never promise timelines or approvals. Rules and review paths change; your project\u2019s facts decide.
Featured builder: JMB Builders LLC
An Altadena-only general contractor (B — General Building) that takes on detached ADUs and garage conversions as real construction, not a side product. Featured placement — disclosed, and verified against the state license record.
Two minutes. A real number. Zero pressure.
Answer seven questions about your lot and plans. We\u2019ll show the honest range for your ADU — and render a concept of what it could look like.
Start the ADU cost quiz