Thinking About an ADU? Start Here, Before You Buy Plans.

Thinking About an ADU? Start Here, Before You Buy Plans

By Matthew Sutherlin | Green Bee Construction Inc | Serving Grass Valley, Nevada City, Auburn, and the Nevada and Placer County foothills

Most people considering an ADU have no idea where to start. They know they want one. A place for aging parents, a rental that helps with the mortgage, room for an adult kid. What they don't know is which questions to ask first, or where the answers actually live.

This is that starting point. A walk through the considerations that come first, in the order they matter, with links to the county resources that hold the information you need. We build ADUs across the foothills, and a recent garage conversion on a 1930s Tudor in downtown Grass Valley surfaced just about every consideration on this list, including a few that catch homeowners off guard.

The video above is a different property, a site walk we did for a client on an unincorporated parcel near Nevada City. It shows the kind of first-look evaluation that starts the whole process. Here's how to walk it yourself.

First, Where Can It Even Go? Zoning and Setbacks

Before anything else, you need to know what your zoning allows and where your setbacks are. Zoning decides how many structures your property can have and whether an ADU is permitted. Setbacks decide where on the lot a structure can physically sit, how far off the side and rear property lines it has to stay.

Put those together and you've answered the first real question: is there a spot where an ADU can legally go, and how big can it be there. Sometimes the answer opens the whole backyard. Sometimes the buildable area is smaller than the homeowner pictured. Either way, you want to know before you fall in love with a floor plan. Nevada County homeowners can use the county's My Neighborhood parcel map to pull zoning, parcel data, and the assessor's map by address, and Placer County keeps a similar assessor parcel map and GIS system. If you'd rather have it confirmed, the planning departments will walk you through what your parcel allows.

New Structure or Convert What You Have?

The next fork is whether you're building a standalone structure or converting something already there, a garage, a shop, an outbuilding.

If you've got an existing structure, the first question is whether it's the right candidate at all. Does it have good bones, a sound foundation, solid framing, a structure that's square and dry. Does it already have utilities reaching it, or is there a pile of infrastructure to build before it's even viable. A conversion can be a real head start. It can also be a teardown wearing a disguise, where the repairs cost more than building new. That's a judgment call worth making early.

A conversion does carry one real advantage: converting an existing structure is generally exempt from the setback rules that apply to new construction. On a tight lot, that can make a conversion work where a new build can't.

Don't Overlook the Junior ADU

There's a third option a lot of homeowners never consider: a junior ADU. A JADU is a unit of up to 500 square feet built inside the existing walls of your house, often a converted bedroom or an attached garage with interior access. Because the shell already exists and there's no new foundation or roof, it's usually the lowest-cost path to a second unit.

The rules here changed recently, and in homeowners' favor. For standard ADUs, California no longer allows a local agency to require that the owner live on the property. For JADUs, a 2026 change (AB 1154) narrowed the owner-occupancy rule so that it only applies when the JADU shares a bathroom with the main house. Give the JADU its own bathroom, and the owner-occupancy requirement generally goes away, which opens up renting both the house and the JADU. JADU rentals do have to be longer than 30 days, so this is long-term tenant territory, not nightly rental.

Here's where it gets interesting for a property's income potential. Under current state law, a single-family lot can hold a standard ADU and a junior ADU at the same time, the JADU inside the home and the ADU as a separate structure. Depending on your lot, your septic, and your budget, that can mean more than one new income source on land you already own. It's worth weighing which combination actually fits your property.

One caution. This is fast-moving law. The state rules above changed at the start of 2026, and counties are still updating their own ordinances to match. Treat this as the direction things have moved, then confirm how Nevada County or Placer County is applying it to your specific parcel before you build a plan around it.

Are the Utilities Close, or Is That a Big Bill?

This is where projects get expensive quietly, and it's the part most homeowners haven't thought through.

Start with power. The ideal answer, financially, is that power runs from your existing main panel and that panel has enough capacity to carry an accessory unit. If it does, you're running conduit a relatively short distance. If the panel is maxed out, you're looking at an upgrade before you start. The more expensive path is a brand-new service, which means checking whether there are transformers on the poles nearby that can feed a new drop to the ADU. In our experience, a new service can run in the range of $15,000 to $20,000, and the bigger cost is often time. A new PG&E service can take six to nine months to get brought over and turned on. Costs and timelines move, so treat those as planning numbers, not quotes. The lesson holds either way: if you need new power, start that process early, because it will be the long pole in the tent.

There's also a billing point most people miss. A separate meter for the ADU makes for a cleaner setup if you're going to rent, because the tenant's utilities are cleanly their own. Worth deciding up front, because it affects how the service gets designed.

Sewer or Septic? This One Question Can Move the Budget $30,000

How you handle waste is one of the biggest cost forks in the whole project, and it has to be resolved as part of your initial considerations, not discovered later. There are really three paths.

If you're on public sewer, you may be able to tie the ADU into the existing sewer line at the house, and that's often the end of it. The cheapest path by far.

If you're on septic, the question is whether your system can carry the added load. In Nevada County, an ADU is required to have its own septic tank, and that tank can tie into your existing leach field only if the total bedroom count between the house and the ADU stays within what the system was designed for, according to Nevada County Environmental Health. Environmental Health reviews and approves this before the building permit is issued, and in most cases requires a repair area kept clear for a future system. The county is direct about one thing worth hearing early: many lots on septic are small and can't accommodate an ADU under current requirements, so they recommend talking to Environmental Health before you go far.

The cost spread is wide. Tying into an existing system you have room on can be relatively modest. A traditional new system might start around $20,000. An engineered system, the kind a tight or difficult lot can force, can run $30,000 to $40,000 fast. That single determination can swing your budget more than almost any design choice you'll make, which is why it belongs in the first round of questions, not the last.

One more thing the brochures skip. On our Grass Valley ADU, we got the whole thing built and then hit a downstream repair where the ADU line connected to the original house. The line running under the original house was a ninety-year-old section of rotted three-inch pipe that had to be dug out from under the original foundation and tied into the four-inch main line. It worked, but it was one more hiccup to resolve before the project was truly finished. Old properties hide these. Budgeting a little margin for what the trenching uncovers is just realistic.

Whether it's sewer, septic, or a well for water, capacity becomes its own question, and that's where the records search comes in.

Where to Find the Answers: The Records Trail

If you don't have your plans, don't know your zoning, and aren't sure what your septic can handle, you are not stuck. The information exists. It's spread across three different offices, and knowing which one holds what saves a lot of wandering.

The building department sometimes has the original building plans for your structures, and permit history on past work.

The environmental health department is where septic records live. If your property is on septic, this is where you may find the original engineering and the system's design capacity, the bedroom count from the section above that decides whether your existing system can carry the ADU.

The recorder's office holds the deeper history, who bought the property and when, and recorded documents going back through the life of the parcel that the other two offices may not have. Placer County's own assessor notes that official information on the size and use of a parcel comes from recorded documents at the recorder, not from the assessor map alone.

For a lot of homeowners, the best first move is simple: do a public records search and walk into the building department and ask for whatever they have on the property you own. Between those three offices you can usually piece together your existing septic system, possible building plans, and the history that tells you what you're working with.

Two resources make this easier. Nevada County homeowners have the county's ADU resource page and the Mother Lode ADU guidebook, a step-by-step tool built specifically for Nevada County with a cost calculator and development checklist. Placer County homeowners have Accessory Dwelling Unit 101, which walks the four-step process and answers most early questions. Both counties also offer pre-reviewed plan sets, a real savings in time and upfront cost for anyone building new.

What an Older Structure Can Hide: A Real Example

Here's why the records search isn't busywork. On the Grass Valley Tudor conversion, the city told us getting the front unit inspected and signed off would be no big deal, a few egress windows, handrails on the stairs, routine stuff. That's not how it went.

The front structure was built as a residence, then partially used as commercial starting in the 1970s, while the upstairs had always been residential. When we went to convert the backyard garage into an ADU, the city saw the front unit had been permitted as commercial with people living in it, and wanted it brought to current code. That kicked off a requirement to build a one-hour firewall between the two stacked units, and they were ready to make us install a fire sprinkler system that would have run around $40,000.

We dug through the city records and found the structure had been residential and commercial simultaneously the whole time, which predated the 2011 law requiring residential fire sprinklers. That documented history is what changed what was required of us. We still did real work, a sprayed fire-rated assembly on the ceiling, fire-rated pads in the outlets, blocking the air chases running through the structure, all certified by an engineer before the building department signed it off. But the records search saved a $40,000 surprise.

The lesson isn't the firewall. It's that when a county or city says a piece of this will be no big deal, that's exactly the moment to look harder. Older homes and any property with a history of commercial use can carry requirements nobody mentions up front. Someone who has been through it knows where those costs hide and how to dig out the records that settle them. That's the difference a guide who has walked it makes.

What It Costs, Honestly

A well-built ADU in the foothills generally runs $350 to $450 per square foot for the construction itself. That can dip toward $300 for a basic box with no real flair, especially if the owner is sourcing materials. It can climb past $500 for vaulted ceilings, pine on the ceiling, wood-aluminum windows, the finishes that make it feel like a home rather than a unit. It comes down to your taste and your property.

Here's the part that trips people up. That per-square-foot number does not include permits, site development, or getting utilities to the structure, and those vary widely. A flat lot with power at the panel and septic capacity to spare is one number. A sloped lot needing a new service, a panel upgrade, and a new septic system is a very different number. Site conditions can move the total more than the building itself does, which is why a real cost picture has to account for them before you commit.

Running the Numbers

Once you've got a feel for the full cost, permits and site work and utilities included, the financing question comes into focus. Some homeowners use a HELOC. Some take a second on the mortgage. Then the math depends on the goal. If it's rental income, you're weighing the monthly return against the cost of the borrowing. If it's housing a family member, the return is measured in years close by rather than dollars. Some people use the rental income to pay down their mortgage faster.

If the numbers work after all of it, you've got a project worth building. If they don't, that's worth knowing now, before anyone's drawn a plan. Financing options designed specifically for ADUs are also starting to emerge, which is widening what's possible for some homeowners.

When to Bring in a Contractor

A contractor can come in at any point, and the right time depends on you. Some homeowners enjoy the research. They'll pull the records, check the zoning, and crunch the loan numbers themselves, and only bring in a builder once they know the project pencils. That's a valid path, and this guide is built to help you walk it.

Others have busy lives and would rather hand the evaluation to a professional. At Green Bee, we do site visits at no cost, and where it goes from there depends on where you are. If you have completed plans and your design questions are settled, we can take those and bid the project. If you don't, this is where a pre-construction agreement comes in, where we help with design coordination, gather subcontractor interest, manage lead times, and build the schedule before plans are finalized.

There's a real reason to involve a contractor during design rather than after. It's called value engineering, evaluating cost as the design develops so you can steer it toward your budget. The homeowners who get burned are often the ones who complete an expensive design in isolation and only then find out it prices them out of their own project. Working through cost during design keeps that from happening.

Why an ADU Is Worth the Work

With building costs where they are, an aging population, and the fact that most people building an ADU already own the property and often already have utilities on site, an ADU is one of the more sensible ways to add a living space. Set aside the income for a second. You're adding a real home to land you already hold.

On that Grass Valley Tudor, the work was significant, jackhammering out the old slab, repairing the foundation, fixing water intrusion through the brick, structural upgrades throughout, repairing old plumbing, and the fire work on the front building. It landed in the top tier of cost. And the rent it generates now pays off the HELOC, funds a reserve for property repairs, and puts a property that had been in the red for years cleanly in the black. That's the long arc of a well-considered ADU.

The Takeaway

If you're seriously considering an ADU, the order is what protects you. Check zoning and setbacks. Decide new build or conversion. Trace your utilities. Pull your records from the building, environmental health, and recorder's offices. Get a real cost picture that includes permits and site work. Run the financing. Then decide.

You can do that legwork yourself with the county resources linked above, or hand it to someone who's walked it before. Either way, knowing the steps and the order is what turns a vague idea into a clear decision.

Frequently Asked Questions

How long does it take to get an ADU permit in Nevada or Placer County?
California law requires a local agency to approve or deny a complete ADU application within 60 days. That's the permit decision, not the whole project. From first conversation to finished unit, plan on roughly 9 to 14 months once you add design, the septic and well evaluations, and construction. Using a county pre-approved plan set can shorten the design and review stretch.

How big can an ADU be, and is there a minimum size?
California allows an ADU as small as 150 square feet, and detached ADUs in this region commonly run up to 1,200 square feet. A junior ADU is capped at 500 square feet because it has to fit inside the existing walls of the house. The real ceiling on your lot is usually set by septic capacity, lot coverage, and setbacks well before the state maximum comes into play.

Will building an ADU raise my property taxes?
The ADU gets assessed and added to your tax bill based on the value of the new construction. The rest of your property is not reassessed under Proposition 13. You're taxed on what you added, not on the whole parcel at a new rate, which is a common worry worth putting to rest.

Are there impact fees on an ADU?
ADUs under 750 square feet are exempt from impact fees under California law. Units of 750 square feet and larger pay impact fees proportional to the primary home. Staying under that 750 square foot line is a real budget lever worth weighing during design, since the savings can be significant.

Can I rent my ADU on Airbnb or as a short-term rental?
Often not on a nightly basis. Nevada County and Placer County regulate short-term rentals separately from ADU rules, and many situations restrict ADUs to longer tenancies. Junior ADUs specifically must be rented for periods longer than 30 days under current state law. Confirm the short-term rental ordinance for your specific area before counting on nightly income.

Is the CalHFA $40,000 ADU grant still available?
No. CalHFA's own page states the most recent round of ADU Grant funding was fully allocated as of the end of 2023, and the agency warns that anyone claiming they can get you an ADU Grant today is running a financial scam. There's legislative interest in refunding it, but no confirmed relaunch. Don't build your budget around it.

Can I build an ADU if my property is on a well instead of public water?
Often yes, but the well has to show it can serve a second dwelling. Placer County requires a supply producing at least one gallon per minute per unit, backed by a recent yield test. A well that has served the main house fine for years can come up short once a second home draws on it, so this is one to check early rather than assume.

Have a question we didn't cover? See our full FAQ page for more answers, or reach out directly.

If you've got a property in Grass Valley, Nevada City, or Auburn and you're wondering whether an ADU makes sense on it, reach out for a straight conversation. We'll help you think it through honestly, and we'll tell you if it doesn't pencil. See our ADU construction services, learn about working with us across the county line as a general contractor in Auburn, or contact Green Bee Construction Inc at 530-575-1700 to set up a no-cost site visit.