3 of the Most Anticipated Accessory Dwelling Unit Code Changes in 2020

By Sean Canning

If you’re like us at Ten Seventy Architecture, you’re eagerly waiting for San Diego’s Accessory Dwelling Unit (ADU) code to be updated to reflect the mandated California State code changes. We’re expecting some big updates to the Companion Unit + Junior Unit sections of the code. Companion Units + Junior Units are terms San Diego uses to define Accessory Dwelling Unit types. Here’s a summary of the top 3 ADU code changes that we are the most excited for + how you can take advantage of them on your property.


This is one of the most anticipated San Diego code updates in the last decade. The State code is already law, but we’ve been patiently waiting* since January 2020 for San Diego’s Development Services Department to publish their draft version. *By patiently waiting we mean emailing + phoning the DSD on a monthly basis.

While it’s still unclear exactly what the City will publish, based on our conversations w/ DSD staff, we have a pretty good idea of what to expect:

“RS Zones (single-family) will allow (1) Companion Unit ADU + (1) Junior Unit ADU per lot. RM Zones (multi-family) will allow up to (2) Companion Unit ADUs per lot.”

This would essentially be an up-zone of all residential lots in San Diego. Single-family lots would essentially be allowed to build triplexes (w/ specific regulation). Multi-family lots would be allowed to build an additional 2 Accessory Dwelling Units beyond what the base density of the zone allows. The previous code capped ADU development to a single Accessory Dwelling Unit per lot w/ denser multi-family lots being disqualified. That means every residential lot will now be able to add at least one additional ADU. Keep in mind most of the City fees for ADUs are currently waived -making them a great bargain.

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The current San Diego municipal code requires Companion Unit ADUs which are attached to a Dwelling Unit [proper] to be limited to 50% of the square footage of that Dwelling Unit. In other words, a 1,200sf house would be limited to a 600sf attached Companion Unit. Detached Companion Units can currently be constructed up to 1,200sf, regardless of the size of the Dwelling Unit.

Obviously, this current code can severely regulate the size of planned Companion Units, especially on tight lots w/ a small existing home. Through our meetings w/ DSD staff we are expecting the updated code to read as follows:

“Companion Unit ADUs, attached or detached, may be a maximum of 1,200sf + may be larger than Dwelling Units on the same lot.”

This code change will open the doors to better develop infill lots + properties w/ small existing homes. For example, you could purchase a 25’x140’ lot w/ a 1,000sf home, then build a 2,400sf duplex behind it. If the lot is super tight, you can attach a Companion Unit to the house + not be limited to 50% floor area (which would be a 500sf ADU in our example).


This code change is going to be a huge benefit for property owners w/ RM (multi-family) lots. If your lot has a density that allows 3 or more Dwelling Units the current code prohibits you from developing any ADUs. Additionally, if your lot allows 2 Dwelling Units, you currently have to build both before you can move onto develop an ADU. This can be significant because City fees for new Dwelling Units can be around $40k while ADUs are generally less than $10k.

Recently we successfully petitioned the Building Department to allow us to permit ADUs on RM lots that only have a single Dwelling Unit (not a maxed out density), citing the State code. This is an example of what the updated code may look like:

“Companion Units may be constructed in all RM (multi-family) residential zones provided there is an existing or proposed Dwelling Unit.”

Considering the huge difference in fees, this can mean significant savings. In multi-family zones this change allows applicants to build up to 2 Companion Units before they have to build Dwelling Units, saving around $30k on each ADU before building the more expensive DUs.

These are our top 3 most anticipated changes to the Accessory Dwelling Unit code. While the California State ADU code changes are in effect now, without an updated municipal code, San Diego is left in accessory dwelling limbo. We’re told a draft ADU code will be issued at the end of July 2020. After that the Planning Commission will review in August, Council Land Use / Housing in September, + City Council will get their eyes on it in October.

The expected changes will be a great improvement + provide opportunities for many homeowners. If you’re interested in designing a custom ADU or would like to discuss the pre-approved ADU design we're developing feel free to contact us for more information.


California State Accessory Dwelling Units code

San Diego Municipal Code: Separately Regulated Uses

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