Ask what an ADU costs and you will hear a number per square foot. That number tells you very little about your project. The real cost of an accessory dwelling unit is a stack of separate lines, and the type of unit you choose moves almost every one of them. This note walks through the four types, the lines in a budget, and what a lot adds before a single wall goes up.
Detached, attached, garage conversion or JADU
California recognizes four ways to add a unit to a single-family lot, and they start from very different places.
Detached ADU. A new building in the yard. It needs its own foundation, its own roof and the longest utility runs of the four. In exchange, it gives the second household the most privacy and gives you the most freedom with the layout.
Attached ADU. New space built against the house, with its own entrance and kitchen. Sharing a wall usually shortens the sewer and power connections, and sometimes the roof framing can tie into the existing structure.
Garage conversion. An existing garage becomes living space. The shell is already standing, which can save a large share of the structural budget. Whether it actually does depends on the slab, the framing and the roof, and an old garage can surprise you on all three.
JADU. A junior unit of up to 500 square feet carved out of the existing home, with its own exterior entrance and an efficiency kitchen. It is the smallest change and usually the lowest cost, because it reuses rooms that already have walls, a roof and finished utilities nearby.
Comparing these side by side for one specific lot is the most useful first step, because the cheapest type in general is not always the cheapest type for your property. Our ADU construction page goes through the same comparison in more detail.
The cost lines: plans, permits, utilities, site work, build
A written ADU budget breaks into five groups. Each one is driven by different things.
Plans and engineering. Drawings, structural calculations and the energy compliance work the state requires. A pre-approved or standard plan can shorten this line. A custom design or a difficult lot lengthens it.
Permits and city fees. Plan check and permit fees are set by the city. State law limits impact fees on smaller units, with units under 750 square feet exempt from them, so the size you choose can change this line more than you might expect. School district fees and utility connection charges are separate and vary.
Utilities. Sewer, water and electrical service have to reach the unit. A short run from the house costs far less than a trench across the yard. If the main electrical panel is already near capacity, a panel upgrade belongs in this line too. A separate meter for the ADU adds cost but can simplify renting it out.
Site work. Grading, demolition of an old shed or slab, access for equipment, and any drainage work the lot needs. On a flat, open lot, this line can be small. On a tight or sloped one, it can be one of the largest.
The build. Foundation, framing, roofing, windows, insulation, finishes, fixtures and appliances. This is the line people usually mean when they quote a per-square-foot figure, and it is the one most affected by your finish choices.
What a Valley lot adds: slope, setbacks, sewer distance
No two lots are alike, in Woodland Hills or anywhere else we work, and three conditions can move an ADU budget more than any finish choice.
Slope. If the lot slopes, a detached unit may need a stepped or engineered foundation, retaining walls or extra drainage. A garage conversion or a JADU avoids most of that, which is one reason the comparison between types matters.
Setbacks. State law allows new ADUs as close as four feet from the side and rear lot lines, but easements, existing structures and the shape of the lot decide where a unit can really go. A good spot on paper can be a poor spot once the trench routes and access are drawn in.
Sewer distance. Wastewater has to flow downhill to the main line. If the unit sits lower than the connection or far from it, a pump or a long trench may be needed. That single detail can make a detached unit at the back of the lot cost noticeably more than an attached one beside the house.
These conditions are why we review the lot before quoting a price.
Why per-square-foot numbers mislead
A per-square-foot figure takes the total cost and divides it by the floor area. The trouble is that many of the costs above do not grow with size. A 400-square-foot unit still needs a kitchen, a bathroom, a sewer connection and a set of permits. Spread across fewer square feet, those fixed costs push the per-square-foot figure up, which makes small units look expensive and large ones look like a bargain.
The same figure also hides the type. A garage conversion and a detached new build of the same size can differ sharply in cost, and a single average across both describes neither.
How we price an ADU in writing
Our process starts with the lot. We look at the parcel, the setbacks, the existing structures and the utility locations, and we tell you which of the four types are possible. From there, we prepare a written budget broken into the lines above, so every dollar sits on a named line and you can tell which choices move it. If a line depends on something we cannot see yet, such as the condition of an old garage slab, the budget says so and explains how we will find out.
If you are deciding whether to add a unit or simply more room for the household, our room additions page compares an addition with an attached ADU, and our garage and structural work page explains what a conversion asks of an existing slab.