Most people start with a picture in their head: a small cottage out back, or the garage turned into a place for a parent. The lot usually decides which of those is realistic. This is how we walk a property when someone asks which kind of ADU makes sense.
The three common types
Detached ADU. A separate building on the lot. It gives the most privacy and the most freedom in layout, and it asks the most of the lot: open space in the right place, a way to get materials and people back there, and utility lines that can reach it.
Attached ADU. An addition that shares a wall with the house, with its own entrance. It can work on lots where the backyard is too narrow for a separate building, but it changes the house itself, so the existing structure, roof lines and the main home's windows all come into the design.
Conversion. Turning existing space, most often a garage, into a dwelling. The footprint is already there, so the site work is usually lighter. The trade-off is that you are working inside a building that was not built to be lived in: insulation, ceiling height, the slab and the electrical service all need a hard look.
What the lot tells you
Walk the property with these questions before you fall in love with a plan.
Where is the open space, and is it inside the setbacks?
Your local code sets how close a new building can sit to the side and rear property lines and, often, to the main house. State law in places like California and Washington limits how strict those setbacks can be for ADUs, but your city still applies its own code. Draw the setbacks on a lot map first. What's left is the area a detached ADU can use.
Can you get to the back?
A detached ADU needs a path for a crew, materials and, later, the person living there. A side yard that's narrow or blocked by a fence, a tree or the AC unit can push you toward an attached unit or a conversion.
Where are the utilities?
Sewer, water and power have to reach the new unit. On a sloped lot, sewer often decides where a detached ADU can go, because the line needs fall toward the main. Septic systems add their own rules. Ask early: this is one of the most common reasons a plan changes after the first site visit.
What about parking?
If you convert a garage, you lose its parking. Many states have reduced or removed replacement-parking requirements for ADUs, especially near transit, but check what your city requires before you plan on it.
The Age-in-Place ADU Guide
Design a home around the life your parent wants to keep, build it so it holds up as needs change, and pay no more than it should cost.
A simple way to decide
- Open, accessible backyard, utilities within reach: a detached ADU is worth pricing.
- Tight or steep backyard, solid house: look at an attached ADU.
- Sound detached or attached garage you don't need for cars: a conversion is often the shortest path.
None of these is always cheaper. A conversion that needs a new slab, new framing and a new electrical service can cost close to new construction. Get a builder to look at the actual building before you assume.
Start with the lot, not the floor plan
Before you pay for drawings, look at your lot lines, the setbacks and where the utilities run. A builder's online lot check can show you a first answer from your address. It is a starting point, not a permit: your city has the final word on what fits.