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Phase 01 of 06

Phase 1: Planning your ADU

Before you sketch anything, you need clarity on three things: whether you can build, what you want it for, and roughly what you can spend. The planning phase is unglamorous and almost free — and skipping it is the single most expensive mistake homeowners make.

Typical duration

4–8 weeks

Typical phase cost

$0–$500 (mostly your time)

What "planning" actually means

The planning phase ends when you can answer four questions confidently. Can I legally build an ADU on this lot? What kind of ADU do I want? What will it realistically cost in my market? And how am I going to pay for it? You haven't hired anyone yet, you haven't drawn anything, and you've spent close to nothing.

Most homeowners want to skip ahead — get drawings, get a quote, get started. Don't. Every hour you spend in planning saves three later. SnapADU, which has built over 100 ADUs in San Diego since 2020, calls out plan revisions and changes after design as one of the seven most common delay drivers — and every one of those changes started as a planning question that wasn't answered up front.

Step 1: Confirm feasibility

Run our feasibility check first — it'll surface the obvious blockers (zoning, lot size, HOA, utilities) in about two minutes. If you come out as "qualified" or "qualified with conditions," the next step is to pull your specific parcel data from the city or county GIS system. Look for:

  • Lot size and setback requirements. Under California state law, detached ADUs under 16 feet tall require just 4-foot side and rear setbacks (California HCD ADU Handbook, March 2026).
  • Allowed ADU types and maximum sizes on your zoning. California allows up to 850 sq ft for studios and one-bedroom ADUs and 1,000 sq ft for two-or-more-bedroom units; many cities allow up to 1,200 sq ft on lots with adequate coverage.
  • Owner-occupancy requirements. California eliminated mandatory owner-occupancy for most ADUs through AB 881; outside California, it varies.
  • Parking minimums. State law eliminates parking requirements for ADUs within half a mile of public transit, in historic districts, in existing structures (garage conversions), or where on-street parking permits aren't required — which covers most California ADUs in practice.
  • Whether your jurisdiction has pre-approved plans. Los Angeles (LADBS) offers 20+ free pre-approved standard plans with permit timelines of 50–70 days. San Jose, San Diego, Berkeley, and Sacramento have similar catalogs.

Most planning departments offer a free 15–20 minute pre-application conversation. Take advantage of it — they'll tell you what's actually buildable before you spend a dollar on design. Multifamily property owners should know that California's SB 1211 (effective 2025) now allows up to 8 detached ADUs on multifamily lots (up from 2), which can dramatically change the calculus for landlords.

Step 2: Define what the ADU is actually for

The purpose of the ADU shapes nearly every later decision. Be specific:

  • Rental income. Long-term tenant, short-term rental, or executive corporate housing? Each has different code, utility, and design implications, and short-term rentals are restricted or banned outright in many California cities. If long-term rental is the plan, the median 2026 California ADU rent ranges from $1,400 in Riverside County to $3,500 in Los Angeles County (Dynamic Quality Builders, 2026), with one-bedroom units the most rentable.
  • Multi-generational housing. Aging parents need accessibility features (zero-threshold entries, grab bars, wider doors) at minimal additional cost if designed in; retrofitting later is expensive. A returning adult child may not need any of that.
  • Home office or studio. Watch local rules — jurisdictions sometimes treat workspaces differently from dwellings, and the design tradeoffs (no kitchen, no bath) can actually push you toward a JADU rather than a full ADU.
  • Future sale value. Resale-focused builds prioritize flexible layouts and broad-appeal finishes. Permitted ADUs add 50–70% of build cost to property value short-term, and 80–100% after rental income is documented (DQB 2026; consistent with Fannie Mae and Freddie Mac 2024 underwriting guidance that permits lenders to factor ADU rental income into qualification).

Step 3: Build a working budget

A working budget has three parts: a target all-in cost (what you want to spend), a stretch budget (what you'd spend if everything goes slightly worse than expected), and a financing plan (how you'd cover either number).

Run our cost calculator for a defensible mid-range estimate. The 2026 California ranges from recent builder cost surveys give you the order of magnitude:

  • Junior ADU (JADU): $50,000–$100,000 (Kellow 2026; DQB 2026)
  • Garage conversion: $80,000–$175,000 California average, $100,000–$175,000 in SoCal specifically. Outside California, Angi reports a national average of $110,000 with a $60,000–$150,000 range.
  • Attached ADU: $150,000–$300,000 CA average, $200,000–$300,000 in SoCal for 500–800 sq ft (DQB 2026; Kellow 2026).
  • Detached ADU: $200,000–$400,000+ CA. Most 800 sq ft mid-finish detached ADUs land $250,000–$350,000.
  • Prefab: $200,000–$400,000 fully installed. Prefab compresses construction time but not site work or utilities.

Then add a 10–15% contingency for overruns and another 5–10% for finish-level drift during construction (Kellow 2026). That's your stretch budget. If you can't comfortably fund the stretch budget, redesign the project before you commit to it.

Don't forget the soft costs most homeowners under-budget: design fees (typically 8–12% of construction cost — Andalusia Drafting 2026 reports professional soft costs of $9,000–$30,000), permits and impact fees (we'll cover the full range in Phase 3), surveys and soils reports ($1,500–$5,000 combined), and Title 24 energy compliance in California ($1,500–$3,500).

Step 4: Decide the order of operations

There are two reasonable paths from here.

Architect-first: hire a residential architect or ADU specialist to design the unit, then bid the design to three builders. This usually nets a tighter design and more comparable bids, but costs more in design fees upfront. The traditional model also adds 4–8 weeks for bidding on top of design.

Design-build: hire a single firm that handles both design and construction. ACI Build, which publishes detailed timeline data for the San Francisco Peninsula, reports that design-build eliminates 8–16 weeks of process time compared with the traditional architect-then-bid sequence. The key reason: when the city sends correction comments during plan check, design-build teams turn them in days; the traditional model takes one to three weeks per cycle because comments have to bounce between architect, contractor, and homeowner.

Most homeowners are best served by design-build for garage conversions and pre-approved plans, and architect-first for fully custom detached ADUs over 800 sq ft where design exploration matters more than process speed.

What success looks like at the end of Phase 1

  • You know exactly what your jurisdiction will let you build.
  • You have a stretch budget you can fund and a financing direction identified.
  • You've chosen a path (architect-first or design-build).
  • You've identified 3–5 potential designers or builders to interview in Phase 2.
  • You've decided what the ADU is actually for and what you'd most regret compromising on.

When all five are true, move on to Phase 2 — Design.

Have questions about your phase 01 decisions?

Get the free Homeowner's Playbook — full breakdown of every phase, with checklists.

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Design