Here is an interesting blog about a custom modern home in Seattle. It was built by a well known architecture firm. It should be able I give you another data point.

http://www.ahousebythepark.com/journal/archive/2010/11/04/final-costs-and-stats-of-the-project/

Finished our home in the area last summer. When we began the architect search in early 2013 we were told $260/ft was about the lower limit, including everything but land. This turned out to be a pretty good estimate in the end. We have a very nice home, but it's not a magazine home.

Since you are starting now, it will be more. $300 seems reasonable for the lower end. But this doesn't get you a whole lot of bling.

Some of it depends on where you are exactly. In the city being more expensive.

Also, what are your expectations about what custom means? Nanawalls and terrazzo floors, etc. vs more standard selections.

There are a lot of costs and we felt nickel-and-dimed to death be the end. Different sites may need topographic survey, geotechnical engineer, permits can be $5,000 or $10,000. Septic, shared well, difficult utilities, etc.

Good luck.

We are building in Magnolia on a relatively flat lot. We are tearing down the existing home so there is already a hole in the ground. We may be able to resuse the foundation but this is TBD.

Do you have a spreadsheet with budget info you'd be open to sharing?

Doesn't hurt to ask!

The architect we worked with employed an estimator and we had a good idea of costs before we went looking for a builder. It costs some extra, but it's recommended.

How builders come to their numbers is somewhat mysterious. We focused on the bottom line and the type of contract offered.

We got 2 ballpark estimates and two detailed estimates. Not that much difference in the bottom lines (8% max?) But the line items showed a lot of variations. My spreadsheets would not be helpful to you.

You might save on site work and foundation which could be significant, but then pay more on labor and overhead. Hard to say.

Good luck!

We are currently doing the owner-builder program through Timberland Bank and are coming in at about $105sq ft - this includes everything down to the architect who designed the plan. We are not in Seattle but a town about an hour south. This is with standard selections (engineered hardwoods, Lowe's Kraftmaid cabinets and quartz counter tops). We're definitely saving a lot doing it ourselves but it is a LOT of work getting bids and doing some of work on our own (luckily the hubby is very handy!). But in the end very worth it to us to design our own home and pick everything we want!

I was also going to estimate 300.00 / sf before I read any of the other responses. Just for construction, not design or permit fees. Prices have gone up a lot since the "house by the park" project.

Check out the Build Blog-- they do very contemporary houses, but they have some good solid information that is applicable to any style and they build their custom houses on challenging lots in the city, frequently teardowns or close to it. But yes, it depends a lot on what your desires are for finishes, etc.