The major difference between detached ADU and attached ADU is the regulations for their maximum size. Yes, governments of different states and local laws of your cities regulate such things.

It is common for many cities in California to allow the construction of attached ADUs like garage additions that are not bigger than half of the square footage of your primary dwelling. In some cases, the restrictions are bigger, so that attached ADUs should not be bigger than 30% of the square footage of the main house and, thus, could only be considered as single-family ADU.

As for the detached ADUs, the rules are not as strict. That way, most jurisdictions in California allow your detached ADU to be as big as 1,200 square feet. All in all, California state laws permit your detached ADUs to be up to 800 square feet, no matter the part of the state in which you are living right now.

Aside from these regulations are also additional ones regarding the height of your Accessory Dwelling Unit. In this case, the upper hand belongs to the attached ADUs: in most cases, detached units cannot be higher than 16 square feet and also will need at least 4 feet of rear and side setbacks each. At the same time, attached ADUs like a garage or basement ADU share a wall with the main residence, which is why their height could be the same as the one belonging to the initial house, and no setbacks are needed.

As you could have already guessed, the pricing of attached and detached ADUs is also quite different. And as expected, a detached ADU will cost more than an attached ADU.

There are multiple reasons for that. First, a detached ADU, in comparison with an attached Accessory Dwelling Unit, does not share a wall with the main house and needs an entirely new foundation. And because of that, an attached ADU in the form of your garage or basement will need a smaller amount of raw materials to be used, and the number of hours spent by the professionals on the construction of the unit will also be lowered.

In the end, a detached unit may cost you around 300,000 dollars, a price comparable to that of a small house; an attached ADU will have a smaller price, somewhere around 200,000 dollars, with the price going down if the ADU is built inside of an already existing structure of the homeowners, such as the attic or the basement.

From the non-legal point of view, an attached Accessory Dwelling Unit that is built not on the rest of the property land but inside of an already existing structure can seem like a simple remodeling project for most families. But as long as your attached ADU project includes a separate entrance, bathroom, and kitchen, the laws of your state will consider it to be an ADU, not a remodeling of an already existing structure.