Building an ADU in Monterey County is less about the building than about the order you do things in. Most delays come from finding out late that the lot is in the Coastal Zone, that the septic system can’t take another unit, or that the property has no water credits. Settle those first and the design and permit are straightforward.
RDD Solutions is a residential design and drafting firm in Carmel. We prepare permit-ready ADU plans across Monterey County. We don’t build and we aren’t an architecture firm. This guide walks through the sequence from first question to issued permit, and what’s specific to building here.
How to Build an ADU in Monterey County: The Short Answer
- Find out which agency has jurisdiction over your parcel.
- Check the site: zoning, Coastal Zone, water, sewer or septic, slope and trees.
- Choose the ADU type: detached, attached, garage or other conversion, or junior ADU.
- Get a survey or as-built drawings of what’s there now.
- Design, or adapt one of the County’s pre-approved plans.
- Add engineering and energy compliance, then submit a complete set.
- Respond to plan-check comments, get the permit, and build with a licensed contractor.
State law requires the reviewing agency to approve or deny a complete ADU application within 60 days, without a public hearing.
Step 1: Find Out Who Reviews Your ADU
Monterey County isn’t one permitting authority. The County’s Housing and Community Development department reviews ADUs in the unincorporated areas, which include Carmel Valley, Pebble Beach, Carmel Highlands, Big Sur, Prunedale and the communities around Salinas. Each incorporated city reviews its own, including Monterey, Carmel-by-the-Sea, Pacific Grove, Seaside, Marina, Del Rey Oaks and Salinas.
Every one of them applies the same state ADU law (Government Code §§ 66310–66342), but local details differ. The City of Monterey, for example, caps ADUs at 850 square feet for a studio or one bedroom and 1,000 square feet for two or more bedrooms. Pebble Beach adds the Pebble Beach Company’s architectural review on top of the County’s permit. Our city pages cover the local process for Monterey, Carmel, Carmel Valley, Pacific Grove, Pebble Beach and Salinas.
Step 2: Check the Site Before You Design
This is the pre-construction planning that decides whether the project is simple or complicated. Check each of these before anyone draws a floor plan.
| What to check | Why it matters in Monterey County |
|---|---|
| Coastal Zone | In the unincorporated Coastal Zone, an ADU needs a Coastal Administrative Permit in addition to the building permit. State ADU law doesn’t waive the Coastal Act. |
| Water | On the Peninsula, new plumbing fixtures bring in the Monterey Peninsula Water Management District, and the property needs enough water credits. Properties on a well or small water system need to show capacity for a new connection. |
| Sewer or septic | An ADU on septic needs its own septic tank and an existing system in good working order. Lots under 1.25 acres on septic typically need an alternative treatment system. |
| Slope and soils | The County’s pre-approved plans assume a level, slab-on-grade site. A sloped lot or poor soils usually means a soils report and a custom foundation. |
| Trees and design review | Protected trees, viewsheds and local design review can limit where an ADU goes, especially in Carmel Valley and the Del Monte Forest. |
| Fire hazard zone | Lots in a mapped fire hazard severity zone need wildfire-resistant construction and may have access requirements. |
On septic, the County’s Environmental Health Bureau reviews the project. On municipal water and sewer, it generally doesn’t. For more on slopes, see when a geotechnical report is needed.
Step 3: Choose the Type of ADU
- Detached ADU. A separate building. Under state law it can be at least 16 feet tall and must be allowed at 800 square feet with 4-foot side and rear setbacks.
- Attached ADU. Shares a wall with the main house, and can generally reach 25 feet or the zone’s height limit, whichever is lower. An attached ADU can also go over an attached garage. See building an ADU over a garage.
- Conversion ADU. Inside existing space, such as a garage or accessory building. No replacement parking is required when a garage becomes an ADU.
- Junior ADU (JADU). Up to 500 square feet inside the main home. The owner must live in either the JADU or the main home.
Not sure what counts? See what qualifies as an ADU in California and the ADU size guide.
Step 4: Document What’s There
Every ADU set starts with an accurate site plan: property lines, existing buildings, setbacks, utilities, trees and slopes. For a conversion or attached ADU, it also needs as-built drawings of the existing structure. Few homes have usable drawings on file, so plan for measuring. A boundary survey is worth getting when the ADU will sit close to a property line.
Step 5: Design, or Use the County’s Pre-Approved Plans
For unincorporated areas, the County offers pre-approved detached ADU plans in two styles:
- One bedroom, 515 square feet
- Two bedrooms, 749 square feet
- Three bedrooms, 1,200 square feet (impact fees apply)
They’re designed for a slab-on-grade foundation on typical soils. You still need a site plan, utility information and a completed ADU checklist, and changes beyond the allowed options turn it into a custom ADU with full plan review. A pre-approved plan suits a flat, open lot. A custom design makes more sense on a slope, a tight lot, in the Coastal Zone, or where the ADU needs to match the main house.
Step 6: Engineering, Energy and the Complete Set
A complete ADU permit set typically includes a site plan, floor plans, elevations, sections, structural drawings and calculations, Title 24 energy compliance, and plumbing, mechanical and electrical information. See what goes into a set of ADU plans.
The 2025 California Energy Code took effect January 1, 2026. A new detached ADU is treated as a new home, so it generally needs solar panels (unless the required system would be under 1.8 kW) and a heat pump for space heating on the prescriptive path. An attached ADU or conversion is treated as an addition instead, with lighter requirements and no solar mandate.
Step 7: Submit and Track the Clock
In the unincorporated county, applications go in through the County’s online permit portal or the permit counter at 1441 Schilling Place in Salinas. Under state law, the agency has 15 business days to say whether the application is complete, and 60 days from a complete application to approve or deny it. Miss the 60 days and the application is deemed approved. In the Coastal Zone, the coastal permit runs at the same time, with no public hearing.
Expect at least one round of plan-check comments. A coordinated, complete set is what keeps that to one round.
Fees to Plan For
- Impact fees: none for an ADU under 750 square feet. Above that, they’re proportional to the size of the main home.
- School fees: generally none for an ADU under 500 square feet.
- Permit and plan-check fees: set by the reviewing agency.
- Utility and water: connection fees, and on the Peninsula, water credits and any MPWMD permit.
For the long-term effect on taxes and value, see does an ADU increase property taxes? and how much value an ADU adds.
Requirements above were checked against County of Monterey and state sources in October 2026. Rules change and vary by jurisdiction, so confirm with your building department before you design. For unincorporated areas, County ADU questions go to 831-755-5025.
Related ADU Guides
- California ADU law in 2026
- Granny flat vs ADU
- Santa Cruz County ADU rules and permit process
- Converting a garage to living space
Monterey County ADU FAQs
How long does it take to get an ADU permit in Monterey County?
State law gives the reviewing agency 60 days from a complete application to approve or deny it. Total time also depends on design, site issues such as septic or slope, and how many plan-check rounds the set needs.
Does Monterey County have pre-approved ADU plans?
Yes. The County offers pre-approved detached ADU plans for one, two and three bedrooms (515, 749 and 1,200 square feet) in two styles. They assume a level, slab-on-grade site and still need a site plan and utility information.
Do I need a coastal permit to build an ADU in Monterey County?
If the parcel is in the unincorporated Coastal Zone, yes. The County requires a Coastal Administrative Permit along with the building permit. It runs at the same time and doesn’t require a public hearing.
Can I build an ADU on septic in Monterey County?
Often, yes. The ADU needs its own septic tank, the existing system has to be in good working order, and lots under 1.25 acres typically need an alternative treatment system. The County’s Environmental Health Bureau reviews it.
Do I need water credits to build an ADU on the Monterey Peninsula?
Usually. Adding plumbing fixtures within the Monterey Peninsula Water Management District requires enough water credits for the property. Check this before you design.
Do I need an architect to design an ADU?
No. For a typical single-family ADU in California, a building designer or drafting firm can prepare the plans, with an engineer providing structural calculations where needed.
Start With a Feasibility Check
Send us the address and what you’d like to build. We’ll check the jurisdiction, zoning, coastal status and site constraints, tell you which type of ADU fits, and prepare the permit-ready plans. Contact RDD Solutions.