From granny flat to backyard cottage to garage conversion — we guide California homeowners from idea to occupancy, without the guesswork.
Cornerstone Capital Development Services is a full-service ADU permit entitlement, development, and construction services company. We built our company — and this comprehensive guide — on the same foundation we build every project: due diligence, stewardship, and value.
Includes: Free preliminary property feasibility assessment · California ADU Guide (10 modules) · Permit & zoning checklist · Regular price $149
Cornerstone Capital Development Services is California's trusted resource for homeowners looking to build an accessory dwelling unit (ADU) — whether that's a detached backyard cottage, a granny flat, a garage conversion, or an in-law suite. We guide California homeowners through every stage of the ADU process: feasibility and zoning review, permit filing, design planning, and full construction management across Los Angeles, San Diego, Riverside, San Jose, and the Inland Empire.
Not sure if your property qualifies? Every guide purchase includes a free preliminary feasibility assessment from our California ADU specialists — so you know exactly what your lot, zoning, and budget can support before committing to a single permit or plan.
15 years of residential expertise — zoning law, permit strategy, contractor vetting, cost control, schedule and budgets — summarized into one authoritative, California-specific guide.
What qualifies, what doesn't, and recent California law changes
Lot assessment, setbacks, utility capacity, feasibility checklist
Detached, attached, garage conversion, JADU
Honest cost ranges, hidden costs, and how to control your budget
HELOCs, construction loans, CalHFA programs, ADU-specific lenders
Step-by-step permit strategy, city-by-city tips, timeline management
Vetting framework, red flags, bid comparison, contract essentials
Phase-by-phase milestones, inspection checkpoints, owner oversight
The most thorough ADU resource available for California homeowners — built on real projects.
Whether you go full DIY with the guide or add expert support along the way, we've designed a clear, straightforward path for every homeowner.
Receive instant access to the complete California ADU Guide — your roadmap to building with wisdom, not guesswork.
Use the property readiness toolkit to understand your lot's potential, zoning, and the right ADU type for your goals.
Go DIY with confidence, or add Cornerstone expert services — permitting, design, consulting, or full turnkey construction.
Add professional support at any stage of your project — from a single consultation to full turnkey construction. Each service is backed by 15+ years of California ADU experience and delivered with the integrity our team demands.
Our former city-planner-led team handles every aspect of your permit submission — research, documents, plan check responses, and final approval coordination.
Custom ADU plans from our in-house design team — floor plans, 3D renderings (if required), structural engineering coordination, and Title 24 compliance, all under one roof.
We handle everything — due diligence, design, permits, and construction — under one fixed-price contract. No finger-pointing, no coordination headaches. You get a key.
Feasibility analysis, zoning research, setback calculations, easement review, and a written report. Know exactly what's possible before you spend a dollar.
A dedicated project manager as your single point of contact — coordinating trades, managing schedules, tracking budget, and keeping you informed every step of the way.
Schedule a private session with senior staff or a senior Cornerstone advisor. Bring your questions, your plans, your challenges — and leave with a clear, actionable roadmap.
See how the Cornerstone ADU Guide stacks up — and when to upgrade to expert services.
| Feature | DIY Only | Guide Only ($79) | Guide + Services |
|---|---|---|---|
| Step-by-step ADU process | ✗ | ✓ | ✓ |
| California-specific zoning checklists | ✗ | ✓ | ✓ |
| Contractor vetting tips | ✗ | ✓ | ✓ |
| Expert permit filing | ✗ | ✗ | ✓ |
| Custom architectural design | ✗ | ✗ | ✓ |
| Licensed GC construction management | ✗ | ✗ | ✓ |
| Integrity & transparency | ✗ | ✓ | ✓ |
Answers from California's ADU specialists — covering cost, permits, eligibility, and the free feasibility assessment included with every guide purchase.
Most California properties zoned for single-family or multifamily residential use qualify for at least one type of ADU following state law changes in 2020 — but the specifics depend on your lot size, setbacks, existing structure, local zoning rules, and utility connections. That's exactly what a feasibility assessment determines. Every purchase of the California ADU Guide includes a free preliminary feasibility review from our specialists, so you know your realistic options before spending anything on permits or plans. Get your free feasibility assessment here →
ADU costs in California range widely depending on type: garage conversions typically run $80,000–$150,000; attached ADUs range from $100,000–$200,000; detached backyard cottages and custom builds can run $200,000–$350,000+. Costs vary by region — Los Angeles and San Jose projects tend to run higher than Inland Empire or San Diego County builds. The California ADU Guide covers current cost benchmarks by ADU type and city, and your included feasibility assessment will give you a realistic budget range for your specific property and goals.
Yes — all ADUs, including garage conversions, backyard cottages, in-law suites, and detached granny flats, require a building permit in California. However, since 2020, the state has significantly streamlined the permitting process: most jurisdictions must approve a qualifying ADU permit within 60 days, and many cities now have pre-approved ADU design plans that skip architectural review entirely. Cornerstone Capital handles ADU permit filing across California, including Los Angeles, San Diego, Riverside, Ontario, Pomona, and San Bernardino County. View our ADU permit filing services →
They're all the same thing with different names. An accessory dwelling unit (ADU) is the official legal term used in California zoning law and permit applications. "Granny flat," "in-law suite," "backyard cottage," and "secondary dwelling unit" are informal names for the same structure. For permitting and financing purposes, always use "ADU" or "accessory dwelling unit" — this is the term your local building department will recognize and the term lenders and appraisers use when valuing your property.
Yes. California law allows ADUs to be rented on long-term leases (30+ days) statewide. Short-term rental rules (Airbnb/VRBO) vary by city — Los Angeles, San Diego, and many Inland Empire cities have specific short-term rental ordinances that apply to ADUs. Rental income is one of the primary financial reasons homeowners build ADUs — a well-positioned ADU in the Los Angeles or San Diego market can generate $1,500–$3,000/month, often offsetting construction financing costs entirely. The California ADU Guide includes a rental income feasibility section by region.
A preliminary feasibility assessment from Cornerstone Capital reviews your property's zoning classification, lot dimensions and setback requirements, existing structure type (eligible for garage conversion or junior ADU designation), utility connection capacity, and any local overlay restrictions that could affect your project. Most homeowners who skip feasibility spend $3,000–$8,000 on plans and permits before discovering a constraint that requires a complete redesign. Our assessment eliminates that risk. It's included free with every California ADU Guide purchase — and it's the fastest, most affordable way to know exactly what you're working with before committing to anything. Get the guide and your free assessment →