Haven't built, but it used to be that the "ten square" house (a square being ten feet by ten feet) was the standard Australian suburban post-War home, so I've seen them and lived in them. Of course, we're dry and hot here for the most part, so there's more life possible outdoors than in a snowed-in winter.
Negatives - you asked for negatives. OK - DON'T tie up the bathroom anytime anyone is using the toilet. To put it positively, have separate toilets. You can have a toilet in the bathroom - good idea if you can afford it. However, it shouldn't be the primary toilet for the house.
If someone starts using the bathroom toilet as the primary toilet, shoot them - makes life simpler all around, and you wouldn't want to live with such an ignorant inconsiderate idiot anyway. Maybe give them a chance in advance, and warn them that if they do it, you'll do it.
I would also definitely build for wheelchair and disabled access - wide doors, wide corridors, shallow ramps rather than stairs wherever possible. This also benefits those of us who have even occasional bad backs, and have to shuffle around then, and can't negotiate tight spaces and tight turns. Likewise grab-handles to rise from a toilet or a chair in the shower. Flat shower floor, rather than a shower cubicle - we'll have to do this soon for my father, who is suffering from arthritic knees and can't readily lift his feet. Surprising resale advantages here, for older couples, disabled people, or for people with an elderly relative.
That also says - upstairs space is cheaper to build and cheaper to heat or cool than all-on-one-level, but people with limited mobility can't use it. Always have a bedroom, bathroom and toilet on the ground floor.
You're talking about a two-person household. OK, then you can afford to have a wash-room rather than a separate bathroom and laundry. Washing machine, dryer if used, access to the outside so you can take laundry directly out to a clothesline, bath and shower behind a curtain. Big low sink for washing vegetables or muddy items. All tiled. Serves as your mud-room, as Mark suggested. Separate toilet, lockable doors, accessible both from here and outdoors, so people don't need to come into the house to use the toilet. Separate basin or tub outside undercover. Do whatever you need to in the way of stopcocks or whatever to stop it freezing if that's a concern overwinter. All the plumbing in one space is economical - have the kitchen sink on a wall adjoining this space.
If possible, have covered space outside this wash-room door. Porch, car-port, whatever. Means you can hang clothes to dry even in wet weather, send children (yours, grand-kids, visitors) outdoors to play even in bad weather, overflow area for a big party or space for a rain-affected barbecue.
Separate storage space is good. Cheaper to build. Maybe a garage with adjoining shed space (workshop, row of secondhand cupboards and shelves, overflow sleeping when you get inundated with guests. Maybe connected to the house by your carport.
Try to design so there is a buffer-space (even if it's only the spare bedroom) for noise between living area and main sleeping area. Even if there's only two of you, it matters if someone's sick, or working shifts, or just had a sleepless night and needs to catch up.
As someone said, always put in provision for much more power than you think you'll need. Put in a LOT more powerpoints now while it doesn't cost much, and make provision for easily pulling more cables. Whether that will be power or phone or computer network or television or what you don't yet know, but you can guarantee you'll need more of some sort of cabled services in a LOT of rooms.
Yes! No matter what anyone else tells you, make sure you have TWO bathrooms! No kidding here. Not necessarily two FULL baths....one full bath with tub and shower plus a half-bath would do it. I also wish we had a real guestroom. We use an enclosed porch but it's really not what's needed. Very un-handy as the "doggie door path" to the dog yard goes right through that room.
