You've measured the back corner of the yard three times. The tape says there's room, and the rendering you saved on your phone says it's beautiful, but every time you look up "ADU rules" you hit a wall of setback tables, floor area ratios, and utility upgrade language written for people who do this for a living. Meanwhile a neighbor two streets over got a garage conversion approved in a few months and a coworker says his plans have been stuck in review since spring.
The confusion is normal. Most of it clears up in a specific order, and the first several checks cost nothing but an afternoon.
1. Measure the buildable envelope before you fall in love with a floor plan
Start with what physically fits. Pull up your parcel on the county assessor map, note the lot dimensions, then walk the yard with a tape and mark the corners with stakes. State law generally allows a detached ADU up to 800 square feet with four-foot side and rear setbacks, so a rectangle 20 by 40 feet needs roughly 28 by 48 feet of clear ground once you account for those margins and eaves.
Now subtract the things you forgot. A block wall footing that extends into the yard. The septic leach field, if you're on one in the outer parts of the city. Mature ash or sycamore roots you don't want to cut. A pool setback. If what's left is smaller than your plan, shrink the plan — do not shrink the setbacks. That's a free check, and it's the one that saves the most money.
2. Find your overhead and underground obstructions
Look up. Then look down. Overhead service drops, the SCE conductors running from the pole to your weatherhead, need clearance from any new roof, and a unit placed under them means either a relocated drop or a redesign. Underground, call 811 for a free utility mark before any design work goes far. It takes a couple of business days and paints your gas, water, and communication lines right on the dirt.
Also find your sewer cleanout — usually a capped four-inch pipe near the house or the property line. Its location and depth decide whether the ADU can drain by gravity or needs a sewage ejector pump, which adds roughly $2,500 to $5,000 plus a lifetime of maintenance. Gravity is always better. Move the building before you buy the pump.
3. Check what your electrical panel can actually carry
Open the main panel and read the number on the main breaker. Many Riverside homes built between the 1950s and 1970s still run a 100-amp main, and a lot of tract homes from the 1980s sit at 125 or 150 amps. An ADU with a heat pump, a range, a dryer, and a water heater typically needs 60 to 100 amps of its own.
If your main is 100 amps and full of tandem breakers, plan on a service upgrade: new panel, new meter socket, utility coordination, and permit. Budget $3,500 to $8,000, more if the service needs to be moved or undergrounded. This is not a DIY check beyond reading the breaker label — anything past the main lugs is live even when the breaker is off, and that work belongs to a licensed electrician.
4. Decide between conversion, attached, and detached
This is the single largest cost lever, so decide it early. A garage conversion reuses an existing slab, walls, and roof, and typically runs $150 to $250 per square foot in the Inland Empire. Detached new construction starts at bare dirt — footings, slab, framing, roof, full utility runs — and typically runs $250 to $400 per square foot depending on finish level and site access.
So a 400-square-foot garage conversion often lands between $60,000 and $100,000, while an 800-square-foot detached unit commonly falls between $200,000 and $320,000. What moves those numbers: a service upgrade, a long trench run to the sewer, a raised foundation on sloping ground, imported fill, tile roofing to match the house, and finish level in the kitchen and bath. Access matters more than people expect. If a mini excavator can't get through the side yard, material moves by wheelbarrow, and labor hours climb.
One position worth stating plainly: don't convert a garage you actually use for parking or storage unless you've solved where that stuff goes. Homeowners regret losing the garage far more often than they regret building smaller in the yard.
5. Understand the permit path and the real timeline
ADU applications in California are subject to a 60-day review clock once the submittal is deemed complete, and that last phrase does the heavy lifting. Incomplete sets get returned, and the clock restarts. A complete set generally includes a site plan, floor plans, elevations, a foundation plan, structural calculations, Title 24 energy compliance documents, and a drainage plan showing where water goes.
Plan on three to six months from first sketch to permit in hand, then four to seven months of construction for a detached unit and two to four months for a conversion. Inspections come in a fixed sequence: underground plumbing and footing before concrete, then rough framing, rough electrical, mechanical, and plumbing, then insulation, then drywall, then final. Each one is a stop-work point. Nobody covers a wall before the inspector signs the card.
School fees, drainage fees, and plan check fees vary by jurisdiction and square footage, and permit costs for a new detached ADU commonly total several thousand dollars. Ask for those to be itemized in your proposal rather than buried in an allowance.
6. Handle grading, drainage, and the base under everything
Water is what quietly ruins backyard buildings, and Riverside makes it easy to ignore. Nine months a year the yard is bone dry. Then a January atmospheric river drops two inches in a day, and the new pad sits lower than the surrounding grade with nowhere for the runoff to go.
The fix is designed in, not added later. You want finish floor above surrounding grade, positive slope away from all four walls at roughly a quarter inch per foot for the first ten feet, and a defined path — swale, area drain, or solid pipe — carrying water to the street or an approved point of discharge. Under the slab, you want engineered fill compacted in lifts, not native soil bladed flat with a skid steer in one pass. Much of the Inland Empire sits on expansive clay soils that swell when wet and shrink when dry, which is exactly how slabs crack and doors stop latching in year three. A soils report may be required, and even when it isn't, it's often money well spent.
7. Vet the contractor before you sign anything
Ask for the license number and look it up yourself on the CSLB website. The lookup shows classification, status, bond, and whether workers' compensation coverage is on file. Then request a certificate of insurance sent directly from the insurer's agent, not a PDF forwarded by the contractor.
Know the norms on money. In California, the down payment on a home improvement contract is limited to $1,000 or 10 percent of the contract price, whichever is less. Payments after that should follow completed milestones — foundation, framing, rough inspections, drywall, final — not the calendar. A written scope should name the actual products: post size, roof material, window brand and series, insulation R-value, panel size.
Warning signs are specific and worth naming. A large cash deposit. No written contract or a one-page estimate with no scope. A suggestion that the project can be built "under permit radar" or as a shed to avoid plan check. A bid that comes in dramatically under three others, which usually means something real was left out — the service upgrade, the sewer run, the grading, the fees. Unpermitted square footage also does not appear in an appraisal, and it surfaces later at escrow.
What can wait and what can't
Some of this is sequencing, and some of it is safety. Interior finishes, cabinet selections, paint, and landscaping around the unit can all wait until framing is up. Nobody needs tile picked out in month one.
What can't wait: anything below grade or behind a wall. Utility locates before excavation, because hitting a gas service is a life-safety event, not a delay. Waterproofing and flashing details before siding. Slab and footing rebar before the pour. Sewer slope before backfill. If you find yourself being asked to approve covering any of those without an inspection sign-off, stop the work that day and call the building department.
If you have an existing unpermitted structure with electrical running to it and you smell hot plastic, see scorch marks at a receptacle, or the lights dim when something turns on, shut off the breaker feeding it and leave it off until an electrician looks at it. Old extension-cord and buried-Romex feeds to backyard buildings are common in older Riverside neighborhoods, and they're a real fire risk.
When to bring in a professional
Do the free checks first: stake the setbacks, call 811, read your main breaker, locate the sewer cleanout, and watch where the yard drains after a rain. Those five things will tell you more about your project's cost than any online calculator.
Call a licensed contractor once any of the following is true. The buildable area is tight enough that a foot in either direction changes the plan. Your main service is 100 amps or less. The sewer cleanout sits higher than your proposed pad. There's standing water anywhere in the yard the day after a storm. Or you have a design in mind and need someone to price it honestly against what the site will actually allow.
New Generation Construction builds new ADUs, home additions, and outdoor living structures in Riverside under CSLB license 910999. The office is open Monday through Friday, 8 a.m. to 5 p.m., and project managers are reachable by phone on weekends.