Attached ADUs in Modesto

A second home that shares a wall with yours, with its own front door, kitchen and bath. Licensed and insured.

Small backyard home (ADU): A grey shed with double doors in a grassy backyard with trees

How is an attached ADU different from an addition?

An ordinary addition makes the house bigger. An attached ADU is a second dwelling: it touches the house but has its own door, kitchen and bathroom, with no shared living space.

What size can it be in Modesto?

State law allows a cap of 50 percent of the main home's floor area, but that cap can never stop an ADU of 850 sq ft, or 1,000 sq ft with two or more bedrooms, as of October 2026. In 2023, HCD flagged Modesto's 50 percent rule on this point, and the city amended its ADU code in late 2024. See our size guide.

Can it be two stories?

Possibly. Government Code §66321(b)(4)(D) allows an attached ADU up to 25 feet tall or the zone's height limit, whichever is lower, which leaves room for a second floor in many zones.

What are the advantages?

Short utility connections. Plumbing and power are steps away.
Yard space kept. Good for lots with a pool, a garden or a narrow rear yard.
Close but separate. Family members can check in without sharing a kitchen.

What are the challenges?

The new roof and walls have to tie cleanly into the old ones. Fire-rated, sound-damping construction is required at the common wall. The free regional plans do not cover attached units, so custom drawings are required.

Is attached cheaper than detached?

It depends on how the new structure meets the old one. Shorter utility runs help, but a complex roof tie-in or a wall that needs rework can erase the savings. Block Renovation's 2026 Sacramento guide shows attached ADUs at $100,000 to $300,000, as of October 2026; we price yours in a written, itemized quote.

Find out if an attached ADU fits

Share the address. We check side yard width, the height limit and the electrical service first.