You do not need an architect for an ADU. You almost always do need an engineer.

Who may prepare the plans

B&P §5537 and §6737.1(a)

For single family dwellings of woodframe construction not more than two storeys and a basement, for buildings of up to four dwelling units on the same basis, and for garages and other structures appurtenant to them, the Architects Act and the Engineers Act both state that they do not prohibit any person from preparing the plans, drawings and specifications.

So for the house, the ADU, the junior ADU and the garage, no architect's licence is required to prepare them. That is the statute rather than a loophole, and it is the reason a residential permit set of this size does not have to carry an architect's fee.

When an engineer stops being optional

B&P §6737.1(b), the second half

If any portion of any structure exempted by this section deviates from substantial compliance with conventional framing requirements for woodframe construction … the building official having jurisdiction shall require the preparation of plans, drawings, specifications, or calculations for that portion by, or under the responsible charge of, a licensed engineer, or by, or under the responsible control of, an architect licensed pursuant to Chapter 3.

Shall require. Not may. Once the framing leaves conventional construction the building official has no discretion, and the documents for that portion have to bear a licensed stamp and signature.

Which projects have already left conventional framing

• Every garage conversion. A garage had a door across most of one wall. Converting it means infilling that opening, or working a bracing system around what is left of it, in a shell that was never built to the code the finished room has to meet. That is the exact condition the section describes.
• Most additions that open a wall. Removing a bearing line and its bracing and replacing them with a beam is not conventional framing.
• Most interior remodels that remove a wall. Same reason.
• New detached ADUs, in practice, because the lateral design and the foundation both have to be shown and a building official will ask for them.

The exemption and the requirement are the same statute. One half is why this work is affordable, the other is why it is necessary.

Where a civil licence covers structural work

B&P §6731

Civil engineering expressly includes framed and homogeneous structures, buildings, or bridges. Residential structural design sits inside that, which is why the drawings and calculations on these projects are prepared and stamped under a California Professional Engineer licence, civil.