Planning guide · updated October 2026
Adding an ADU on a Monterey County lot starts with two questions most owners skip: is the parcel in a city or in the unincorporated County, and is it inland or in the coastal zone? The answers decide which ordinance applies, how large the unit can be and which permits you need. Then comes water, which on the Monterey Peninsula can matter more than any setback. Here is how the County rules work in 2026 and what we check first. Questions? Call 805-712-8993.
The short answer
In unincorporated Monterey County, inland ADUs follow County Code Section 21.64.030 (up to 1,200 sq ft, junior ADUs up to 500 sq ft), and coastal-zone ADUs follow Section 20.64.030, with a 1,000 sq ft cap in Big Sur and B-8 areas. State law controls where it gives more. Water and septic approval often decide the project. Call 805-712-8993.
1. Who sets the rules
City limits and the coastal zone come first
Monterey County has twelve incorporated cities: Carmel-by-the-Sea, Del Rey Oaks, Gonzales, Greenfield, King City, Marina, Monterey, Pacific Grove, Salinas, Sand City, Seaside and Soledad. Each one adopts its own ADU ordinance and runs its own permit counter, so a lot inside city limits in Carmel-by-the-Sea, Pacific Grove, Monterey, Salinas or Del Rey Oaks is outside the scope of this guide. In Carmel-by-the-Sea, design review also applies; our Carmel design review guide covers that process.
Everything else is unincorporated County land, handled by the County’s Housing and Community Development department (HCD) in Salinas. That includes Pebble Beach and the Del Monte Forest, Carmel Valley, Carmel Highlands, Big Sur, the Santa Lucia Preserve and rural areas such as Prunedale and Aromas. A Carmel or Monterey mailing address does not mean the lot is inside a city, so we always check the parcel itself.
The County keeps two zoning codes. Inland parcels follow Title 21, and its ADU rules are in Section 21.64.030, adopted in 2020. Coastal-zone parcels follow Title 20, and its ADU rules are in Section 20.64.030, rewritten by Ordinance 5432 in November 2025. The County’s Land Use Regulations page lists the coastal regulations as effective December 19, 2025. Most of Carmel Valley is inland; Pebble Beach, Carmel Highlands, the Carmel coast and Big Sur are in the coastal zone.
State law sits on top of both. The inland section says that where state ADU law and the County text conflict, state law controls. The state rules are now in Government Code sections 66310 to 66342, and the California Department of Housing and Community Development (also called HCD) explains them in its ADU Handbook (March 2026 update) (we confirm per lot).
2. What the state guarantees
The floor every ordinance has to meet
Under Government Code section 66323, as HCD reads it, a lot with an existing or proposed single-family home can have three kinds of units approved ministerially, without a hearing or design review:
- A converted ADU inside the existing or proposed house or an existing accessory structure, with exterior access. A conversion of an accessory structure may grow up to 150 sq ft to make room for a stair or entry.
- A junior ADU (JADU) of up to 500 sq ft of interior livable space inside the walls of the house. An attached garage counts as part of the house for this purpose.
- A new detached ADU of up to 800 sq ft with 4-foot side and rear setbacks and a height of 16 feet, or 18 feet near qualifying transit, with 2 more feet allowed to match the roof pitch of the house.
HCD says these can be combined, so a single-family lot can have one of each. Other ADUs, such as a larger detached unit, fall under the local ordinance, which may set size limits but must allow at least 850 sq ft, or 1,000 sq ft for a unit with more than one bedroom.
A few other state rules matter in practice:
- No owner occupancy for ADUs. A local agency cannot require the owner to live on site for an ADU (other than the rare separately sold unit). For a JADU, the owner must live on site only if it shares a bathroom or other sanitation with the house, under the law in effect January 1, 2026.
- Rentals. ADUs and JADUs may be rented for terms longer than 30 days. Short-term rentals are not allowed.
- Parking. No parking can be required for conversions, for units within a half mile walking distance of public transit, or in several other cases. Where parking is allowed, it is capped at one space per unit or bedroom, whichever is less, and a garage converted for an ADU does not have to be replaced.
- Fire sprinklers. An ADU does not need them unless the main house does.
- Timelines. The permitting agency has 15 business days to tell you whether the application is complete and 60 days from a complete application to approve or deny it. A coastal permit for an ADU has its own 60-day clock.
- Homeowner associations. CC&Rs cannot prohibit or unreasonably restrict an ADU or JADU, though reasonable design standards can apply. That matters in gated areas.
3. Inland rules
Section 21.64.030: Carmel Valley, the Salinas Valley and other inland areas
For inland parcels where ADUs are allowed, Section 21.64.030 sets these local standards on top of state law:
- Size. ADUs up to 1,200 sq ft; JADUs up to 500 sq ft.
- How many. The County text limits a single-family lot to one ADU and one JADU. Because state law now allows a converted ADU, a JADU and a small detached ADU together, we ask the County which combination it will approve on the lot before design starts.
- Height. An attached ADU follows the height limit for the house. A detached ADU can be as tall as the zoning allows for a habitable accessory structure or 16 feet, whichever is greater. Going higher than that needs a Use Permit from the Zoning Administrator.
- Setbacks. Four feet from side and rear lot lines, including over a garage. A garage converted, or torn down and rebuilt as an ADU in the same footprint, needs no setback.
- Design. The ADU should look consistent with the house and the neighborhood. The County says a design approval, tree removal permit or other review can still apply depending on the zoning overlay.
- Use. Rentals must be longer than 30 days, vacation rentals are not allowed, and the ADU cannot be sold separately from the house.
- Disasters. If the main house was destroyed or damaged in a natural disaster, the County may issue the ADU building permit first.
Resource-constrained areas. The inland section prohibits new ADUs in areas with water, sewer or traffic limits: parcels with a B-8 district overlay, the North County planning area outside the Castroville Community Plan, a part of the Toro planning area and a part of the Greater Salinas area that the General Plan limits to the first home on a lot, and specific plan areas unless the plan allows them. Where state law still requires an ADU, the County caps a new detached unit at 800 sq ft and 16 feet with 4-foot side and rear setbacks, and conversions must stay inside the existing house or accessory structure plus up to 150 sq ft for access. JADUs are allowed in every inland area, including these. If your lot is in Prunedale, Aromas or another North County community, this is the first thing we check (we confirm per lot).
Older permits carry over. A unit the County approved as a senior citizen unit or caretaker unit counts as an ADU, and a permitted guesthouse can be converted to an ADU under the current rules, even in the restricted areas.
4. Coastal zone rules
Section 20.64.030: Pebble Beach, Carmel Highlands, the Carmel coast, Big Sur and North County
The coastal section, Section 20.64.030, was rewritten in late 2025 and now tracks state law more closely. Highlights:
| Where the lot is | County rule for a new ADU (summary) |
|---|---|
| Inland unincorporated areas, such as Carmel Valley and most of the Salinas Valley | Section 21.64.030: up to 1,200 sq ft; JADU up to 500 sq ft; generally an allowed use, though design approval or tree permits can apply |
| Inland resource-constrained areas (B-8 overlay, most of the North County planning area outside Castroville, parts of the Toro and Greater Salinas areas, specific plan areas) | New ADUs restricted; where state law requires one, a new detached unit is capped at 800 sq ft and 16 ft high with 4-ft side and rear setbacks. JADUs are still allowed |
| Coastal zone: Carmel Area, Del Monte Forest (Pebble Beach) and North County land use plans | Section 20.64.030: new ADUs and newer accessory-structure conversions up to 1,200 sq ft; Coastal Administrative Permit unless exempt |
| Coastal zone: Big Sur Coast land use plan and any B-8 area | Up to 1,000 sq ft, and no ADUs inside the Big Sur Critical Viewshed |
| Any of the twelve cities | The city’s own ADU ordinance and permit counter, not the County |
Summarized from Monterey County Code Sections 21.64.030 and 20.64.030, read October 16, 2026. Overlays and plan-area boundaries are mapped parcel by parcel, so the County’s zoning for the lot decides which row applies (we confirm per lot).
- Permits. A Coastal Administrative Permit is required, processed without a public hearing within 60 days of a complete application. An ADU or JADU attached to or fully inside an existing house may be exempt, unless the lot falls under one of the location exceptions in the Coastal Commission’s regulations. Other parts of a project, such as tree removal or work on steep slopes, can still trigger a full Coastal Development Permit.
- How many. The County text allows one ADU and one JADU on a lot with an existing or proposed single-family home, and more on lots with multifamily buildings. As inland, we ask the County how that squares with the state’s three-unit combination for the specific lot.
- Size. 1,200 sq ft in the Carmel Area, Del Monte Forest and North County land use plan areas; 1,000 sq ft in the Big Sur Coast plan area and B-8 areas. The caps apply to new attached and detached ADUs and to conversions of accessory structures built after March 25, 2024. The County text also limits an ADU carved out of the main house to half of the existing house.
- Height. A detached ADU can be 16 feet, or 18 feet within a half mile walking distance of a major transit stop or high-quality transit corridor, plus 2 feet to match the roof pitch. More height for architectural consistency takes a Coastal Development Permit.
- Setbacks and coverage. New ADUs meet the front setback of the zone and need 4 feet at the sides and rear. Conversions and same-footprint rebuilds need no setback. Site coverage and floor area limits apply, but they cannot block an 800 sq ft ADU with 4-foot setbacks.
- Parking. One off-street space per ADU only on lots in the areas mapped in Appendix 15 of the Coastal Implementation Plan; the County publishes a list of parcels subject to ADU parking. Elsewhere, none, and a converted garage does not have to be replaced.
- Design. Where the zoning includes a “D” design control district, the Chief of Planning issues a design approval, and that decision is not appealable.
- Big Sur. ADUs are prohibited in the Big Sur Critical Viewshed. Our Big Sur permits guide explains how that viewshed is tested.
- North County coast. To protect groundwater, every structure on the lot needs low-flow fixtures and Energy Star appliances, and landscaping has to avoid permanent irrigation, before an ADU permit is issued.
- JADUs. Up to 500 sq ft with an efficiency kitchen, a separate entrance and an interior door to the house if the bathroom is shared, plus a recorded deed restriction. The coastal text still asks the owner to live on site; state law since January 1, 2026 requires that only when sanitation is shared, so we confirm which rule the County applies to your plans.
Pebble Beach lots also go through the Pebble Beach Company’s architectural review and Del Monte Forest tree rules; see our Pebble Beach building permits guide.
5. Water
On the Monterey Peninsula, water can decide the project
Much of the Monterey Peninsula, including Pebble Beach and parts of Carmel Valley, is served by California American Water (Cal Am). There, the Monterey Peninsula Water Management District (MPWMD) issues the water permit that comes before a County building permit. MPWMD’s ADU water permit page says adding an ADU follows its normal water permit process:
- The permit is based on the fixture units the ADU adds, using MPWMD’s permit information sheet.
- An instant-access hot water system is required, with no credit given for it.
- Adding a bathroom triggers retrofits on the whole property, including high-efficiency toilets and low-flow showerheads and faucets, and no water credit is given for those retrofits.
- Some permits need a recorded deed restriction, which adds a few days.
The bigger question is supply. Condition 2 of a 2009 State Water Resources Control Board order bars Cal Am from diverting Carmel River water for new service connections or for increased use at an existing address caused by a change in zoning or use. MPWMD applied in October 2025 to lift that condition, and as of October 16, 2026 the State Water Board’s hearing was still under way, with public comment set for November 5, 2026. Whether an ADU on a given lot can get a water permit depends on the property’s water history and any credits or entitlements, so we ask MPWMD before design (we confirm per lot).
Wells and small water systems. If the ADU will draw from a private well or a water system with 2 to 199 connections, the County Environmental Health Bureau (EHB) must sign off. For ADUs, EHB checks both water quality and quantity; for JADUs, quality only. Its Environmental Health Bureau ADU checklist asks small systems of 2 to 4 connections to show at least 3 gallons per minute for each proposed connection, and source capacity and water quality tests generally must be no older than five years. If water supply or sewage disposal is not adequate, the County will not allow the unit.
6. Septic
Small lots often need an upgraded system
If the lot is on septic, EHB reviews the system against the County’s Local Agency Management Program (LAMP). Its guidance for ADUs:
- EHB recommends a dedicated system for each dwelling. The ADU should have its own septic tank, though it may share the dispersal field with the house.
- The existing system must be in good working order, shown by an OWTS Performance Evaluation, and the lot needs room set aside for a future dispersal field repair.
- If estimated nitrogen loading exceeds the limit for the parcel, a supplemental treatment unit is required. The County says that in most cases, properties under 1.25 acres on septic will need an alternative system with supplemental treatment to add an ADU.
- All horizontal setbacks from wells, slopes and water features in the LAMP still apply.
EHB does not routinely review ADUs that connect to public water and sewer, unless the site has a well, a graywater system or rainwater catchment. You can ask EHB whether it has a septic record for the property before you start, which helps size the decision early. Whether an existing system can take an ADU is a site question (we confirm per lot).
7. ADU or guesthouse
Two different things in County code
A guesthouse in Monterey County is sleeping and living space with no kitchen, not to be rented, and tied to the main house by a recorded deed restriction. Inland, Section 21.64.020 limits size and holds a guesthouse to 15 feet and one story. In the coastal zone, Section 20.64.020 caps a guesthouse at 425 sq ft of livable floor area and 12 feet. Neither can be rented.
An ADU has a full kitchen, can be rented for more than 30 days and is usually allowed to be larger. For most owners who want flexibility, an ADU is the better fit; a guesthouse can make sense where an ADU is restricted. An existing permitted guesthouse can be converted to an ADU under the current rules.
Our garage conversion vs detached ADU guide compares conversions with new detached units, and our ADU cost guide covers what moves the budget.
8. County pre-designed plans
Three ready-made ADU plans
The County’s ADU Plans and Info page offers three pre-designed detached ADU plans in two styles, “Santa Lucia Modern” and “Ventana Ranch”: one bedroom (515 gross sq ft), two bedrooms (749 gross sq ft) and three bedrooms (1,200 gross sq ft). Things to know:
- The plans still go through the County’s normal review for each lot. You need a site plan showing where the unit goes.
- The foundations assume a slab on grade with presumptive soil values. Sites that do not meet those conditions need a soils report and a custom foundation design, which is common on Peninsula hillsides.
- The County notes that the three-bedroom plan requires payment of impact fees. Under state law, ADUs of 750 sq ft of interior livable space or less are exempt from impact fees, and larger ones are charged in proportion to the main house.
- Changes beyond the listed options turn the plan into a custom ADU with a new plan review.
A custom design often fits a sloped or wooded lot better. Either way, a new detached ADU must meet the California Energy Code, including solar for a new conditioned detached unit; our Title 24 energy code guide explains those rules.
9. The permit path
From parcel check to final inspection
- Confirm jurisdiction. City or unincorporated County, and inland or coastal zone.
- Check overlays and area rules. B-8, design control, the Big Sur Critical Viewshed, ADU parking areas and any HOA or community design review.
- Settle water and septic. MPWMD water permit on the Cal Am system, EHB review for wells, small water systems and septic.
- Design. Size, height and setbacks to the rules above, plus fire code: our fire hazard severity zone guide covers construction standards in mapped zones.
- Submit. Apply through the County’s online Accela Citizen Access portal or at the HCD permit counter at 1441 Schilling Place in Salinas, with the coastal permit or exemption checklist if the lot is in the coastal zone.
- Plan check and corrections. The 15-business-day completeness notice and 60-day decision clock apply under state law.
- Build and inspect. Inspections through final, then the unit can be occupied.
Buying a home with an unpermitted ADU? State law limits when a permit for an existing unit built before January 1, 2020 can be denied over building code issues, unless the problem threatens health or safety. That can make legalizing an older unit realistic; we look at the structure first.
How we help
ADUs in Monterey County with us
- A parcel check before plans. We confirm city vs County, inland vs coastal, overlays, water and septic so the design starts from what the lot allows.
- Plans that fit the lot. Sister studio Harrod Drafting & Design draws custom ADUs or adapts a layout to a sloped or wooded site, and coordinates soils and energy reports.
- One team through construction. Our design-build process keeps design, permits and the build in one conversation, and we manage inspections to final.
- Outside, too. Sister company Paso Robles Landscaping handles landscape and hardscape, including the low-water planting some areas require.
Harrod Built is a family general contractor (CSLB #986950) based in Paso Robles that builds across Monterey County. See our ADU and guest house page, and for lots up the road in San Luis Obispo County, our ADU rules for San Luis Obispo County. You can verify any contractor with the CSLB license check.
Thinking about an ADU in Monterey County? Send us the address and a sentence on what you have in mind: a new detached unit, a garage conversion or space inside the house. We will check whether the parcel is in the coastal zone, which area rules and water source apply, and what that means for size and permits before anyone draws plans. Call 805-712-8993 or contact us.
Sources: Monterey County Code Sections 21.64.020 and 21.64.030 (inland zoning), Sections 20.64.020 and 20.64.030 (coastal zoning, as amended by Ordinance 5432) and Chapter 21.42 (B districts), read from the County’s official Municode library (Supplement 56); County of Monterey Housing and Community Development ADU Plans and Info page and Land Use Regulations page; County Environmental Health Bureau page “Can I Build an Accessory Dwelling Unit on my Property?”; California Department of Housing and Community Development (HCD) Accessory Dwelling Unit Handbook, March 2026 update, which summarizes Government Code sections 66310 to 66342; Monterey Peninsula Water Management District water permit and ADU pages; State Water Resources Control Board California American Water Company page and June 19, 2026 hearing notice. All checked October 16, 2026. This guide is general information, not legal advice. State ADU law changes often, and we confirm current requirements with the County for each lot.
Can I build an ADU in Monterey County?
On most residential lots in unincorporated Monterey County, yes. Inland lots follow County Code Section 21.64.030 and coastal lots follow Section 20.64.030, with state law setting the minimum. New ADUs are restricted in some inland areas with water, sewer or traffic limits and in the Big Sur Critical Viewshed, and water or septic approval can still stop a project.
How big can an ADU be in Monterey County?
Inland, up to 1,200 square feet, and up to 500 for a junior ADU. In the coastal zone, up to 1,200 square feet in the Carmel Area, Del Monte Forest and North County plan areas and 1,000 square feet in Big Sur and B-8 areas. In restricted inland areas, a new detached unit is limited to 800 square feet.
Do I need a water permit for an ADU on the Monterey Peninsula?
On the Cal Am system, generally yes. The Monterey Peninsula Water Management District issues a water permit based on the fixture units the ADU adds, with an instant-access hot water system and whole-property retrofits as conditions. Whether water is available for a given lot depends on its water history and the State Water Board order on Cal Am, so check before design.
Do coastal ADUs in Monterey County need a coastal permit?
Usually a Coastal Administrative Permit, decided without a public hearing within 60 days of a complete application. An ADU attached to or inside an existing house may be exempt unless the lot falls under a Coastal Commission location exception, and other work such as tree removal can require a full Coastal Development Permit.
Do the County rules apply in Carmel-by-the-Sea or Monterey?
No. The twelve incorporated cities, including Carmel-by-the-Sea, Monterey, Pacific Grove and Salinas, each have their own ADU ordinance and permit process. County rules apply only in unincorporated areas such as Pebble Beach, Carmel Valley, Carmel Highlands, Big Sur and the Santa Lucia Preserve.
Planning guides
Budget and planning guides
ADU cost on the Central Coast
Detached, attached or conversion: what moves the price of an ADU or guest house.
Garage conversion vs detached ADU
Setbacks, parking, size, utilities, energy code and fire rules for converting a garage versus building a new detached ADU.
ADU rules in San Luis Obispo County
County vs city rules, how many ADUs a lot can have, size, setbacks, water, septic and fire checks, pre-reviewed plans and the permit path.
Pebble Beach building permits
Del Monte Forest coastal permits and design approval, Pebble Beach Company ARB, protected trees, Cal Am water and fire rules.
Building in the Santa Lucia Preserve
Design Review Board stages, CSD water and will-serve, lot-specific fuel management and Monterey County permits.
Carmel design review: Track 1 vs Track 2
Carmel-by-the-Sea design studies: staff-level Track 1 vs Planning Commission Track 2, site assessment, story poles, trees and volume.
Big Sur building permits and the critical viewshed
Coastal permits, staking and flagging, the Highway 1 viewshed test, water and septic, fire access and rebuilds on the Big Sur coast.
Building in a fire hazard severity zone
What Chapter 7A (and the California Wildland-Urban Interface Code) change on the roof, vents, walls, windows, decks and landscape.
ADU rules in Santa Barbara County
Unincorporated County lots: the two ADU paths, size and height, Montecito and coastal permits, water, septic, fire and where state law wins.
Building in Carmel Valley
The Master Plan, County design review, slopes and grading, wells, septic, Cal Am water and fire district rules.
Start your project
Planning an ADU in Monterey County? Tell us about the lot.
A few details help us call you back with useful answers about budget, schedule and permits.
805.712.8993michael@harrodbuilt.com
1345 Riverside Ave, Paso Robles, CA 93446
CSLB License #986950
