Skip to content
OwnAdu

Phase 05 of 06

Phase 5: ADU construction

With permits in hand and financing in place, your builder mobilizes within two weeks. From foundation to drywall to finishes, this phase runs 12–40+ weeks depending on ADU type and site. Most cost surprises happen here — and most are preventable with discipline.

Typical duration

12–20 wks (garage conv) · 16–28 wks (attached) · 18–32 wks (detached) · 28–40+ wks (hillside)

Typical phase cost

60–75% of total project cost

How long construction actually takes in 2026

ACI Build's 2026 timeline data, validated against SnapADU's reporting on 100+ San Diego ADUs, gives the realistic construction-phase ranges by ADU type:

  • Garage conversion: 12–20 weeks. Fastest, because the shell exists. You're closing in the space, installing MEP, insulation, drywall, finishes, and kitchen/bath.
  • Attached ADU: 16–28 weeks. Foundation work (potentially different from the primary home), framing, MEP, finishes — plus the complexity of tying into the existing home's systems and managing work on its envelope.
  • Detached ADU on standard lot: 18–32 weeks. Clear the site, pour the foundation, frame, install MEP, insulate, drywall, finish. The variables that move the high end of the range are soil conditions and utility distance.
  • Hillside or complex foundation ADU: 28–40+ weeks. Geotechnical testing, retaining wall design and construction, pier foundation drilling, grading, drainage infrastructure. Common in California's coastal foothills.

The 11-step construction sequence

SnapADU's complete construction guide (April 2026) breaks the build into 11 sequenced steps. Every ADU follows roughly the same sequence; timing varies by type and complexity.

  1. Mobilization (within 2 weeks of permit). The ADU footprint is staked on the property. Site fencing, temporary toilet, signage, and a dumpster go in. Initial site prep needs (foliage, rocks, fill) are scoped.
  2. Site work (1–3 weeks). Clearing, excavation, grading, utility trenching. Garage conversions skip most of this. Unexpected soil conditions discovered here can trigger geotech re-assessment.
  3. Trenching & foundation (2–4 weeks). Footings, slab, or pier system. Concrete cure is the critical-path item; you can't frame on green concrete.
  4. Framing (3–6 weeks). Floor system, walls, roof frame, sheathing. This is when the ADU starts to look real.
  5. Rough MEP (3–5 weeks). Electrical wiring, plumbing supply and drainage, HVAC (heat pumps and mini-splits have become standard for ADUs per SnapADU's HVAC guidance). Followed by city rough inspections.
  6. Exterior finishes / "water tight" (3–5 weeks). Roof shingles or membrane, windows and exterior doors, siding or stucco. Once the building is water-tight, interior work begins in parallel.
  7. Insulation & drywall (3–4 weeks). Insulation per Title 24 (California), drywall hang, tape, mud, sand, prime. Some homeowners take over here under a "shell" or "tape and texture" handoff.
  8. Interior finishes (6–10 weeks). Flooring, cabinets, counters, tile, fixtures, paint, appliances. The longest interior phase by far.
  9. Final MEP and exterior (2–3 weeks). Trim-out of MEP, final exterior elements, hardscape.
  10. Final inspections (2–4 weeks). Building, electrical, plumbing, mechanical finals. See Phase 6.
  11. Punch list & closeout. Cleaning, touch-ups, punch-list completion, certificate of occupancy, and (optionally) landscaping.

Inspections happen at every phase boundary

Cities inspect at several milestones: foundation pre-pour (rebar and forms), framing (after windows and rough MEP), rough MEP (often combined with framing), insulation, and final. Missing or failing an inspection delays everything downstream — your builder is responsible for scheduling, but you should know the sequence so you can spot trouble.

The seven most common delay drivers

SnapADU explicitly enumerates the seven delay categories they see most often, in declining frequency:

  1. City inspection backlogs. Building inspectors are on their own schedule. In high-volume markets, framing, rough MEP, and final inspections can each be delayed by weeks.
  2. Coastal review. Properties in the California Coastal Zone (Carlsbad, Oceanside, Encinitas, parts of San Diego) need CDP approval. Even with AB 462's concurrent 60-day mandate, coastal projects move slower than inland.
  3. HOA approval cascades. Many HOAs have specific ADU rules. Submit construction documents to the HOA at the same time as the city so feedback can be incorporated in parallel.
  4. Utility upgrades and septic. Electrical panel upgrades ($2,500–$6,000), new sewer connections ($1,000–$15,000), and septic system replacements add unplanned time. Map existing utilities before construction.
  5. Right-of-way work. Undersized main lines or driveway changes can require right-of-way permits that take weeks to obtain, with multi-agency coordination.
  6. Plan revisions and changes. Every mid-build design change triggers a permit revision, additional design fees, and downstream rework. Make decisions during design and stick with them.
  7. Material lead times. Custom cabinetry, specialty tile, and certain windows run 12–20-week lead times. Identify and order during permitting, not during framing.

Change orders: the silent budget killer

Most cost overruns come from change orders during construction. A change order is a written, priced amendment to the original contract. Two non-negotiable rules:

  • Nothing changes without a written change order. "We can do that, just slide me an extra $5K cash" is how you end up in court.
  • Every change order resets the schedule. Even small changes have downstream effects — accept that or stop changing things. SnapADU describes this as the construction schedule's "waterfall effect": when one trade falls behind, every subsequent trade pushes too, and because subcontractors are booked across multiple projects, you may wait weeks for the electrician to return.

How to be a good client during construction

  • Weekly walkthrough. Same day each week, with your superintendent. Take photos. Write down questions. Ask them at the walkthrough, not over text at 9 PM.
  • Approve invoices promptly. Builders run tight cash cycles. Slow approvals turn into slow subcontractors next month.
  • Stay off the job site between walkthroughs. Especially during framing — it's not safe and you're not helpful.
  • Document everything in writing. Texts and emails beat verbal agreements. Photos beat both.
  • Plan for weather. SnapADU explicitly addresses the common worry that rain during framing damages the structure; modern construction methods handle this safely. Wet framing lumber is normal and dries; the concern is unprotected interior finishes, which builders sequence to avoid.

When things go wrong

Construction will surprise you. Common issues: a subcontractor flakes, a material is back-ordered, the plumbing inspector wants something the designer didn't anticipate, weather delays a critical pour, an appliance shows up damaged. None of these are project-killing if the builder communicates early. A red flag is a builder who goes quiet for more than a few days. Escalate before it gets worse — SnapADU publishes a guide on contractor red flags that emphasizes silence as the strongest warning sign.

Once construction is substantially complete, you move to Phase 6 — Inspection and occupancy.

Have questions about your phase 05 decisions?

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

By submitting, you consent to OwnAdu and up to three vetted ADU builders contacting you by email about your project. Consent is not a condition of purchase. Unsubscribe at any time via the link in any email. See our Privacy and Terms.

← Previous phase

Financing

Next phase →

Inspection & occupancy