There are five types of ADUs: detached, attached, garage conversion, junior, and above-garage. They run from about $20,000 for a junior ADU to $360,000 for a detached build, and from 2 months to 12. Your lot size and your budget usually pick the type before preference gets a vote.
There are five ADU types, and the difference between the cheapest and the most expensive is about $340,000. That is the whole reason this page exists. Most people arrive having already decided they want a backyard cottage, and a third of them leave having realized their garage was the better answer.
The five are: a detached ADU standing on its own in the yard, an attached ADU sharing a wall with the house, a garage conversion built inside a structure you already own, a junior ADU carved out of the house itself, and an above-garage ADU stacked on top of the garage.
Here is the shortcut I give homeowners. Cost and lot space pick the type far more often than preference does. If your budget is under $100,000, you are looking at a conversion, and the only question is whether it is the garage or a spare bedroom. If you have real yard and real budget, detached wins on rent and on appraisal. Everything in between is a trade between what you own already and what you are willing to give up.
The table below is the honest version of that trade, with every number pulled from the same cost engine that powers our ADU cost calculator, so what you read here and what the calculator quotes you will agree.
Compare every ADU type
Cost, size, timeline, and permit complexity, side by side.
| Feature | Detached ADU | Attached ADU | Garage Conversion ADU | Junior ADU | Above-Garage ADU |
|---|---|---|---|---|---|
| Typical cost | $120,000 to $360,000 | $95,000 to $200,000 | $55,000 to $140,000 | $20,000 to $100,000 | $100,000 to $280,000 |
| Typical size | 400 to 1,200 sq ft | 400 to 800 sq ft | 300 to 650 sq ft | 150 to 500 sq ft | 400 to 800 sq ft |
| Typical timeline | 6 to 12 months | 5 to 10 months | 3 to 6 months | 2 to 5 months | 7 to 12 months |
| Permit complexity | Moderate | Moderate | Low | Low | High |
| Best for | Owners who want maximum privacy and a fully independent living space | Owners who want an ADU without giving up backyard space | Owners on a budget who already have an underused garage | Owners who want rental income fast at the lowest possible cost | Owners with a detached garage and little extra lot space to build on |