If you have a special property, you want to find an architect to design a special house and not a generic plan factory house. Get exactly what you want in your new build.

A good architect will probably have builders they have worked with in the past that are quality builders.

And most importantly, start reading. Read the threads here. Read books on good design. Read books on working with a builder. Read books on what constitutes good home design.

Edmonds...good luck keeping costs down.

I would say figure 2 years and have a very healthy budget.

First start with architects who specialize in eco friendly. Lots of those in seattle.

You need a team.

Concurrently find 1-2 recommended builders to discuss cost factors of the project, including demolition and permitting as well as the permitting and approval process. If you're in a neighborhood and/or municipality that has a design review process, get to know it. Understand that costs will continue to change while you go through the process, especially if you're a year+ from breaking ground.

I agree with others here. You are very unlikely to find plans online that would suit your situation. I like Jeffrey's idea of putting together a design-build team.

You can do some homework and preparation in advance of meeting with an architect or builder. Get a good survey that your architect can use to tailor the design to your land. Familiarize yourself with local zoning ordinances to find out height limits, setbacks, size limitations, historic preservation requirements etc. Put together a wishlist and start thinking about what you like and don't like functionally and stylistically.

First step is getting a sense of what your budget is.

Next - this is a tricky topic here. This board believes STRONGLY in going the architect route. We are doing that, and there was never any question that we wouldn't. BUT it is definitely more expensive. And definitely worth it. But people have different budgets. If you are dealing with a reasonable non-shoestring budget, then yes - DEFINITELY hire an architect.

But whether to go to a builder or architect first - there are mixed opinions on this. If a friend has a recommendation for an amazing builder or architect, you should look into them. (If you really think you are going to go with a stock plan instead of an architect - which again, I don't advise - you definitely should interview and choose a builder first. They WILL have specifications on what they will and won't do. If you are looking at eco-friendly, guaranteed if you have a builder who does this without a custom architect it is because s/he builds a very specific design. You're wasting your time and money looking at plans that your preferred builder may not be willing to build.)

Otherwise - you say you want eco-friendly. That is not going to be hard to find in Washington State. But it is still something that only select builders and architects do well. If you want to go as energy efficient as possible, you are talking Passive House. Which is expensive. However, it gives you a good framework to start looking for builders. You don't have to go to the expense of pursuing full Passive House requirements. But this will narrow down your search specifically to builders who build eco-friendly homes. Again, this only matters if you are serious about eco-friendly, as in this is the area you would prefer to focus your budget on, as opposed to tile and molding and cabinets and the like. If you are, this is not something a regular builder does. Or at least does well, in my experience. There is a huge difference between a builder who thinks foaming all the walls constitutes a well-sealed home, and a builder who does blower door tests to ensure they have plugged as many tiny leaks in the envelope as they can find.

Architecture also plays a big part in designing an eco-friendly home, and there are architects who specialize in it. Again, search for passive house architects or green building architects. (Personally, I would look at builders first if you are SERIOUS about eco-friendly. Sometimes green builders do things in very particular ways, such as SIPS or modular, and you would be better served finding that out first and choosing an architect they work well with.)

You will get a good sense of what is out there. Narrow down a list of professionals, and then contact them. You will probably rule several out over the phone, just based on budget requirements and other issues. Then meet with whose left and go from there. They will definitely have answers to your listed questions. And for questions like timeline, those answers will vary quite a bit between builders, so will perhaps play into your hiring decision.

We don't know how much you can do, you'll have to judge that.
But you should avoid a diy design or stock plan. That's the first and most crucial place to get good professional help. ie work with an architect. It is a significant cost, and a significant benefit.
You mention things like parents and grown kids. These are the type of things that a good architect can take into account, and things that stock plans don't.
Architect. Trust us one this one :) And along with that, their recommended contractor.

Timeline? Can't say. But longer than anyone expects, predicts, hopes, and prays.

But have you thought this through fully? Why break down a house bringing in rental? Why not look for land and build afresh, keeping the existing property?

How do I find where my plot map is and will it show the measurements of my plot?

It is possible your town website will have this information. My town's website does, and we're not particularly populous here.