Since we decided we wanted an ADU (apartment, see needs), figuring out where to put it was our first concern in layout.  Size wise, we wanted it to be a one bedroom unit, of about five to six hundred square feet: plenty for one person and workable for a young couple. Since our space requirements looked to be about 1500 square feet, that meant the ADU would be about one quarter of the building.  ADU's are often in basements, but since the building had no existing basement, and even if it did, the lot is very level, so getting light into the basement would be hard.  Another alternative is on the second floor, but that led to complications due to both the code requirement that the ADU door be not in front, and that the ADU would occupy only half a floor.  If the stairs to the ADU would be inside the house, we'd either need two sets of stairs, or a way to share a stairway.  Two sets of stairs would waste too much space, and although we could locate our offices upstairs and the main house downstairs, we just couldn't come up with a good layout with shared stairs (maybe we didn't try hard enough?).  Another alternative was to use external stairs, but we didn't see that as being aesthetically appealing so we didn't pursue that path.  Essentially it was easier to put the ADU on the main floor.

Along with placing the ADU, we worked on what the footprint of the house would be.  Based on passive solar principals, the house should be long on the E-W axis, and preferably not too deep on the N-S axis.  This is also good for getting good daylight into the house.  Unfortunately the existing house was 30'6" on the E-W and 32' on the N-S, and zoning laws would only let us go 8 more feet in the E-W direction: one foot in the front and seven feet in the back.  We had no desire to extend to the N due to wanting to avoid shading the neighbor as much as possible, nor did we want to extend to the S, since that is the best outdoor space on the lot.  With the ADU and our public space occupying the main floor, extending seven feet out to the W made the most sense, and otherwise trying to stay within the footprint.

With the footprint decided (including leaving the existing porch), the ADU ended up running E-W on the north side of the main floor, with no real southern exposure.  While this is certainly not ideal (and maybe we will regret it someday), it made everything fit and kept us from having to sacrifice sunlight in our part of the house.  Renters always seem to get sub-optimal conditions.  To compensate, we first added a loft to the ADU on the east face, with a south window looking over the porch, but when we decide to have both a guest room and a guest bathroom, the loft disappeared and we replaced it with a big bay window on the east, allowing for a little south light (conveniently, code allows bay windows to project up to two feet beyond the required setback).  Later, we would use windows and skylights to make sure the ADU got the best light in the situation.  More disappointing was that there would be no way the ADU could get any passive solar gain.

This now left our part of the house with minimal space on the main floor and almost too much space on the second floor (which is partly to explain why the guest room, guest bathroom and art studio got added).  Also when we went to layout the ADU, the space didn't accommodate a decent kitchen, so we ended up putting its kitchen in a 7'6" by 10' bump out to the north, which we put a lower slope shed roof on, hopefully making it virtually invisible to the neighbor (whose floor is about three feet higher than ours.  Also they have a six foot fence on the property line).

To see the final first floor plan click here.
To see the final second floor plan click here.

Ignoring the problem of placing the ADU, the design patterns that affected us most were:

• Separation of public and private space: not only did we want an emotional barrier between the two spaces, we wanted a sound barrier as well.  This led to an interesting dilemma: we both wanted to be able to work in our offices and be isolated from whatever was going on in the kitchen or living room, so having offices up in the private space was good for that, but it also meant that inviting people to come work with us in either of our offices could be a little awkward.  Luckily, we figured that would be rare, and since we created a mini-office with a networked computer on the main floor, we could hopefully use it for that sort of thing. We also separated the guest area upstairs from our private space, although the tradeoff was that Kim's office would have to be part of our private space. 
• Connection to outdoors: we wanted both the ADU and our house to have a good connection to outdoors both in public and private space, with the public space facing the streets and private facing the side yards.   While we'd rather not use obscure glass, we can't deny we have a small lot in a city.  We've seen too many windows in houses that have shades drawn all the time for privacy reasons, and we don't want to create this situation.
• Outdoor space as rooms: we decided it was important to keep the existing porch (about 6'6" by 15') even though in Seattle's climate, a covered east facing porch is too cold for our taste most of the year.  Still, we like the concept and saw its potential.  The south facing space next to the living/dining/kitchen, especially up against the house can be quite warm almost any time the sun it out, makes the best outdoor living space, and so our intent is for it to have a couple of small patios connected by beds of low maintenance planting beds, possibly with a deciduous vine (like wisteria) on a trellis over part.  On the north and west we build a serenity garden focusing on low growing, shade loving plants with lots of evergreen.  This would act as the outdoor space for the ADU bedroom, and as a transition, marking the outdoor boundary between the ADU and our part.
• Sunny Place: although our existing house has good daylight, it doesn't have much of a southern exposure, and we found that not only did we gravitate toward the south living room window on sunny day, but everyone who came to visit did also. We felt this sunny place was an important and so we bumped out a spacious window seat (more like a Hawaiian Pune- a window bed) in our living/dining area to be our sunny place.
• Public bathroom: we located the main floor bathroom in the back of the kitchen, next to the back door.  Since people like to be discrete going into a bathroom, a hallway would probably have been better, but with limited space we had few options.  By being near the back door, it is easily accessible from outdoors.  The guest bathroom is at the top of the stairs, so it close to public space.
• Mail sorting: our mail always ends up on the kitchen counter, the stairway and in the living room.  By creating an office off the living room, we could have various in and out boxes and contain the clutter.
• Size of rooms: the private rooms in our house are rather small by current standards- more like what was common in the 1950s, but to make the house feel bigger we made the living/dining/kitchen area one big room with only a counter to divide the kitchen from the rest.
• Placement and amount of windows: about half of the windows are on the south side so that we take advantage of passive solar gain.  We ended up with more windows on the east face (about 25% of the total), partly because that's the front of the house, and partly because the ADU needed extra windows to compensate for the lack of a south exposure.  The west windows in bedrooms have high sills for privacy, and the north side only has windows where there was no other option.

It was important to us that the house "feel good".  This may be a strange concept to some, but we believe that there is a feel to a house, and anyone who is willing to pay attention to it can tell the difference.   Although we believe the house will feel good, we won't really know until we've lived in it for a while.  Design is all about anticipating the future, and how you use your current house may not be completely indicative of how you will use another house.  Like all designs, we made a lot of compromises, and there were things we wish we could find a better way to do.  But at some point, you have to stop thinking about it and just build it (besides, the architectural fees get really expensive).