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Phase 02 of 06

Phase 2: ADU design

Design is where your project becomes real on paper. Done well, it gives you clean drawings that permit smoothly, builders can bid accurately, and you live in happily for decades. Done poorly, it generates expensive change orders for the rest of the project.

Typical duration

4–10 weeks (pre-approved plans), 8–14 weeks (fully custom)

Typical phase cost

$8,000–$30,000 in professional soft costs

The three paths to a design

You have three reasonable options, in order of increasing custom-ness (and cost).

  1. Pre-approved plans. California cities are systematically adding ADU plan catalogs to streamline permitting. Los Angeles (LADBS) offers 20+ free standard plans and pushes those through plan check in 50–70 days versus 8–16+ weeks for custom designs (Andalusia Drafting, 2026). San Jose, San Diego, Berkeley, and Sacramento have similar programs. You pick a plan, customize it lightly, and design fees stay minimal — Angi's 2026 data puts ADU drafting fees at $2,000–$6,600 for a complete plan set in this model. The tradeoff is limited choice. The catalog plans are competent but they weren't drawn for your specific lot.
  2. Prefab and modular. Manufacturers like Villa, Cover, Plant Prefab, and Connect Homes have configurable models you adjust and order. Design fees are bundled into the unit price. Crucially, site work, foundation, utilities, and the local permitting still happen the same way as stick-built — so prefab compresses construction (often to 3–4 months on site) but not the overall timeline. ACI's 2026 data shows the realistic prefab all-in stays in the 10–18 month range alongside custom builds.
  3. Custom design with an architect or ADU-specialist designer. Most appropriate for detached ADUs over 700 sq ft, sloped or tricky lots, and homeowners with strong aesthetic preferences. Fees are typically 8–12% of construction cost per AIA standards and consistent with what California ADU designers quote in 2026. Andalusia Drafting reports complete custom permit-ready plan sets (architectural + structural + Title 24 + MEP) at $9,000–$30,000 in 2026 California pricing.

The design milestones to lock

Whether you're customizing a catalog plan or commissioning a custom design, you'll move through the same milestones. ACI's 2026 timeline guide breaks the design phase into four substeps:

  • Site assessment (week 1). Designer visits, evaluates setbacks, slope, access, soil conditions, existing structures, electrical panel capacity, and sewer lateral location.
  • Feasibility and schematic design (weeks 2–3). Bubble diagrams, rough floor plans, massing. You're deciding the overall shape and flow. For pre-approved plans, this phase is shorter (1–2 weeks); for fully custom, longer (3–4 weeks).
  • Design development (weeks 4–6). Wall thicknesses, fenestration, structure, kitchen and bath layouts. If a geotech report is required (hillside lots), it's ordered now and takes 2–4 additional weeks to deliver, blocking structural engineering.
  • Construction documents (weeks 6–10). Permittable drawings with full dimensions, schedules, sections, specifications, structural calcs, energy calcs (Title 24 in California), and MEP plans. Builders bid off these.

The further along you are, the more expensive changes get. SnapADU notes that even thorough plans see one round of plan-check revisions from the city — but homeowner-initiated changes after construction documents are issued cost full redraws and push your schedule by weeks. The cheapest time to change your mind is week 2. The most expensive is week 8.

What you should actually decide during design

The decisions that have the highest leverage on the finished home:

  • Building orientation and entry sequence. How the ADU relates to the main house determines privacy for both parties and the rentability of the unit. Plan two entry paths — one for tenants, one shared with the main house — even if you'll only use one initially.
  • Kitchen and bath locations. Plumbing chases drive framing and foundation work. SnapADU explicitly recommends placing kitchen and bathroom on a single plumbing wall as one of their top cost-saving design moves.
  • Window placement. Free if decided in schematic design; expensive if changed later because of structural cascading and Title 24 energy compliance recalculations.
  • Ceiling heights and roof form. A vaulted ceiling costs 6–10% more than a flat ceiling but transforms the feel of a small unit. Worth it almost universally on units under 700 sq ft.
  • Storage. Small ADUs without thoughtful storage feel smaller than they are. Build it in now or regret it later.
  • Long-lead items. Custom cabinetry, specialty tile, premium fixtures, and certain window manufacturers run 12–20-week lead times. Identify these during design so they can be ordered during permitting — material lead times are one of SnapADU's seven most common project-delay drivers.

Working with your designer (don't be a bad client)

The most successful clients show up to design meetings with examples (Pinterest, Houzz, screenshots from real homes), keep decision-making to two or three stakeholders maximum, and return feedback in writing within 48 hours. Slow client decisions are the most common cause of design schedule slips — both SnapADU and ACI flag this as a primary timeline risk.

Be honest about your budget. A good designer will steer you away from decisions you can't afford. Hiding the budget number means your design gets value-engineered later in the worst possible way — by your builder, mid-construction, when every "savings" is actually a change order.

The deliverable that ends Phase 2

A permit-ready set of construction documents, plus any required engineering (structural, Title 24 in California, soils report if needed for hillside lots). For a custom California detached ADU in 2026, expect this package to total around $15,000–$25,000 in professional fees by the time it's ready for submission (Andalusia Drafting 2026). Pre-approved plan adaptations can come in under $10,000.

Once that set is locked, you're ready for Phase 3 — submitting for permits.

Next: Phase 3 — Permits.

Have questions about your phase 02 decisions?

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