In April 2020, the Las Virgenes Municipal Water District board took up a request from a homeowner on Laura La Plante Drive in Agoura Hills. She was asking the district to waive an additional $4,134.50 sewer service capacity fee tied to her property, citing financial hardship. The board said no. The fee stood.
That single line item is a useful place to start, because it shows what actually drives the cost of adding a unit to a property here. It isn't the floor plan. The City of Agoura Hills will hand you one of those for free.
What the city actually gives you for free
Agoura Hills runs a Pre-Approved ADU Plans Program built around three unit types: a 550 square foot studio, a 750 square foot one-bedroom, and a 1,000 square foot two-bedroom, each available in Cottage, Ranch, or Spanish architectural styles with a choice of slab-on-grade or raised foundation. The architecture and planning firm RRM Design Group prepared the sets, and any homeowner can download them at no cost from the city or pick them up at City Hall.
The upside is real. Because the plans are already reviewed and approved, the city generally waives the building plan check fee that a custom design would otherwise require. That saves time and design cost up front.
What it does not do is settle the rest of the permit file. The program's own materials are direct about this: using a pre-approved plan does not remove the need for a permit, and it does not eliminate separate review for grading, drainage, septic systems, other utilities, or fire sprinklers where they apply. Those are the line items that actually separate a $40,000 backyard studio from a $120,000 one, and they depend entirely on the parcel, not the plan.
The line the free plan glosses over: sewer or septic
Las Virgenes Municipal Water District supplies water and treats wastewater for roughly 70,000 residents across Agoura Hills, Calabasas, Hidden Hills, and Westlake Village. Most lots in those cities connect to its sewer system. But Agoura Hills has pockets, most visibly in Old Agoura, where lots were never brought onto the municipal line and still run on private septic.
The city's own ADU guidance splits cleanly here:
| If your lot is on... | The ADU process runs through | What that typically adds |
|---|---|---|
| Sewer (most of the city) | Las Virgenes Municipal Water District for a connection permit and fee | A defined fee schedule and a district review |
| Septic (parts of Old Agoura and other rural-character lots) | City Building and Safety Division and the LA County Department of Environmental Health | A second agency, a septic design review, and a longer timeline |
The Laura La Plante Drive case shows what the sewer side of that table can cost even after a fee waiver request. On the septic side, there is no comparable flat number, because a county health department review depends on soil conditions, tank capacity, and drain field placement specific to the parcel. That variability is exactly the point: two Agoura Hills addresses using the identical pre-approved floor plan can face entirely different approval paths before a shovel goes in the ground, and neither is visible on the listing sheet.
Three more line items the free plan doesn't cover
Beyond wastewater, three more requirements attach to the lot rather than the plan.
Solar. Under the 2022 California Energy Code, newly built detached ADUs need a photovoltaic system, with narrow exceptions for very small structures. If the main house already has solar, the ADU's system can sometimes tie into it rather than requiring a standalone array. Mobile home ADUs are excluded and fall under a different state code.
Power source. Agoura Hills adopted an Energy Reach Code as part of its Climate Action and Adaptation Plan, and pre-approved ADU plans are drawn as all-electric with no gas infrastructure permitted. The citywide reach code does carve out one relevant exception: an attached ADU or junior ADU tied to an existing home that already has gas infrastructure is not required to go all-electric. A detached unit does not get that exception.
Fire and grading. State law requires fire sprinklers in an ADU if the existing house has them or is required to have them, and that means a separate plan submitted to the Los Angeles County Fire Department for its own review and final inspection. On larger, rural-character lots, if any part of the proposed structure sits more than 150 feet from the street property line, a fire apparatus access road becomes part of the design. And once grading passes 50 cubic yards, a licensed engineer has to sign off before the city issues a grading permit.
None of these show up in the free plan set. All of them show up in the county and utility files behind the lot.
The $40,000 grant almost everyone still expects
A lot of ADU cost math still gets built around the CalHFA Accessory Dwelling Unit Grant Program, which once reimbursed homeowners up to $40,000 in pre-development costs, things like design, permits, and soil tests. It is a reasonable number to plan around because it circulated widely for a couple of years.
It is also not available right now. CalHFA's own program page still carries the same notice it posted in December 2023: the latest round of funding was fully allocated, and no new round has been announced. If you are penciling out an Agoura Hills purchase on the assumption that this grant will offset your ADU's soft costs, that assumption needs to be checked directly with CalHFA before it goes into a budget, not sourced from an older blog post that never got updated.
There has been real movement elsewhere in the process this year. As of January 1, 2026, state law gives cities 15 business days to tell an applicant whether an ADU permit application is complete, which tightens a step that used to drag out with no clock on it. And a 2023 federal mortgagee letter lets lenders count a portion of an ADU's actual or projected rental income toward a buyer's qualifying income on certain purchases, which matters if the ADU is part of how you plan to afford the house in the first place. Neither of those is a check in the mail, but both change how predictable the process is compared to a few years ago.
Before you write the offer
If an ADU is part of why a particular Agoura Hills property makes sense to you, a few questions belong in your due diligence before contingencies come off, not after:
- Ask the seller's agent directly whether the lot is connected to Las Virgenes Municipal Water District sewer or runs on private septic, and request any septic permit or pump history if it applies.
- If septic is involved, build in time for a Los Angeles County Department of Environmental Health review alongside the city's, since that is a second agency with its own schedule.
- Check whether the existing house has a working solar array or gas infrastructure. Either one can change what your ADU is required to install on its own.
- If the lot is large or the house sits well back from the street, ask whether a fire apparatus access road would be triggered for a detached unit.
- Confirm the current status of any state or local ADU funding program directly with the agency before it becomes part of your financing plan.
A few questions worth asking directly
Does every ADU in Agoura Hills have to be all-electric? The pre-approved city plans are drawn all-electric with no gas infrastructure. Under the citywide Energy Reach Code, an attached ADU or junior ADU connected to a home that already has gas service is exempted from that requirement. A detached unit is not.
Is the $40,000 CalHFA grant coming back? There is no announced date. The program's own page still shows the funding as fully allocated since December 2023, and any planning around it should start with a direct check of that status rather than a secondhand summary.
Does the pre-approved plan mean I skip permitting entirely? No. It removes the building plan check review and its fee. Grading, drainage, septic or utility connections, and fire sprinklers, where applicable, still go through their own review.
If you are weighing an Agoura Hills purchase with a future ADU in mind, the plan is the easy part. The lot is where the real budget lives, and it is worth walking through before you write an offer, not after you own it. Nancy Cassidy has spent 25 years working these Agoura Hills and Old Agoura streets and can help you read a specific parcel before you commit to one. Let's Connect — Schedule a Consultation.
