Do you actually need an ADU — or just a well-designed guest wing?
TLDR: On a waterfront property in Pierce County, an ADU — even an attached one — pulls your entire project into a 100–170 day shoreline review. A well-designed guest wing gets you the same privacy and separation while staying on the faster permitting path.Just because you can, doesn’t mean it's the best option for your property.
Why Everyone Thinks They Want an ADU
The appeal is obvious - a guest house gives you separation. Your brother in law can stay over without changing your daily routine. Your parents can move in and give you a stay at home date night. Now that ADUs are widely allowed across Washington State, it's become shorthand for “smart planning” to include one.
“That’s where what’s actually feasible for your property looks very different from ‘allowed per zoning’.”
The Plan That Almost Never Works — Building the ADU First
On large projects, we often consider phasing buildings. A popular idea is to build the ADU first, get occupancy, then build the main house. On a waterfront property, that doubles not just the regular permitting process, but also the Shoreline Review. If the project phasing hasn’t been designed fully, you could also be building yourself into a corner you can’t get out of.
Going through the rigamarole of multiple agency reviews isn’t on anyone’s bucket list. Phasing can make sense when the scale of work is large enough to warrant breaking it up over the years, but in most cases when clients look at the big picture and see how long the process will take, they opt to do it all at once.
What an ADU Actually Triggers on a Waterfront Site
Pierce County has different designations for waterfront development, and a very narrow reading of what is exempt from the 6 month review period for “Shoreline Substantial Development”. Their zoning language is very clear - an ADU, whether it is attached or detached - is outside of the bounds of that exception. They have a line in the code explicitly saying so to be sure there is no confusion. So, in plain language, if you are adding an ADU (attached or detached from the home) your entire development is subject to the “Substantial Development” review, which is subject to a 100-170 day review period, as opposed to 65 days for a new house without an ADU.
What You Actually Need vs. What an ADU Provides
It pays, then, to ask yourself, what do you really want out of this ADU? Is it separation and a way for houseguests to come and go without disrupting the main household, or rental income? A well designed guest wing with its own entrance delivers on the former while staying in the single family designation. Getting in your home 1-2 months sooner usually is worth it.
The Grandfathered Setback Problem
Even if there appears to be a straightforward garage to ADU conversion, if that structure doesn’t already meet the zoning guidelines, you won’t be able to turn it into an ADU. That’s where what’s actually feasible for your property looks very different from “allowed per zoning”.
What Feasibility Work Finds Before You Commit
None of this is hypothetical. The ADU question, the setback conflict, the permitting pathway; these are exactly the kinds of things that surface during a proper site review before design starts. Not during schematic design. Not when the permit comes back. Before anyone draws a single line. Designing whatever you want and waiting till it goes to permitting to work out the kinks is a recipe for redesign and unnecessary delay.
On a waterfront site, that review covers the shoreline designation, the exemption eligibility, the grandfathered status of every existing structure, and the sequence of consultants you'll need before the county will even look at your application. It also covers what happens to your timeline if any one of those factors goes the wrong way.
The homeowners who skip this step don't find out about the 170-day review because they asked the wrong question. They find out because they submitted a permit application with an ADU on it.
If a guest wing gets you everything you need — same privacy, same separation, same function — and costs you six fewer months and tens of thousands less in permitting, what exactly are you getting from the ADU designation?