How about a green roof?

Structural Insulated Panels have bending strength to spare; they make it possible to put a layer of soil on them and grow things on the roof. Such a roof adds insulation and improves stormwater runoff: it captures water for later evaporation and it filters the water that does run off. The soil actually protects the roofing membrane from UV radiation and physical damage.

One study says rainwater runoff is reduced by 75%. According to Wikipedia, "water is stored by the substrate and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation. Green roofs not only retain rainwater, but also moderate the temperature of the water and act as natural filters for any of the water that happens to run off."

We're talking about 660 SF of green roof and 2,265 SF composition shingles.

As the project gets more real, it is important to force myself to continue to dream big and not get bogged down by practicalities. Also, I used to think that buyers would be attracted by an "architect-designed" house. Now, with the hydronic heating system and advanced stormwater diversion, I think the appeal shifts to being a cutting edge (the LEEDing edge, if you will), high-efficiency green house.

Another study says a green roof could increase the value of the house by 7%. Academic bullshit aside, I think it might be a win in the Seattle marketplace. As a practical matter, no one knows what it costs. "Yes, I bought the house for under $500,000, but then I installed...the Future!...and now I want $1 million. Any other questions?"