The home addition isn't necessarily expensive--the price is just substantially higher than you were led to believe by your architect. If you want a reasonably accurate estimate of the cost to design something then by all means, ask designers and architects. They do that for a living But, if you want a reasonably accurate estimate of the cost to build something, please don't be misled into thinking an architect will provide the answer. Look instead to someone who does that for a living--a builder or remodeler, for example.

From what you can tell the master addition is $100,000... that does seem high. About $235/square foot. I know contractors are swimming in jobs right now. I think sometimes they quote outrageously high because they're not interested in the job. Or, they know they're high demand and you'll pay. Just my guesses.

That is an interesting website, live_wire_oak. We're a bit lower end finishes than the mid range master, but sizewise pretty comparable. I downloaded the data for the local region (Mountain, Boise: right state, 300+miles off) and adjacent (Pacific, Spokane, WA: wrong state, in the neighborhood geographically). Looking at prices, the Pacific region is probably more accurate. It was very eye opening and I wish I'd come across this at the outset.

We were hoping to avoid financing, so my husband and I will need to have a serious discussion about phasing it, maybe starting with porches and saving up for the master, and possibly explore costs for building within the garage, bringing it down to a two car 25x26, which is not our favorite option although there's room to add a 16x36 3rd bay eventually.

If it makes you feel better, our bids for an addition of a master suite without a new foundation but with 2 additional bathroom remodels ran from $310k to $475k. We're in a high cost area just outside a major East Coast city. So by comparison, you're getting a great deal!

Yikes, smalloldhouse_gw! Your low end is probably what my whole house would go for.

100k for a 420sq ft master addition sounds about right. There are a lot of fees that people don't think about. Permits, achitect fees, extra plumbing, extra this, redo that.

The quotes did not include architect fees, and one didn't include permit fees. It was minus exterior paint, and flooring which we already had and could self install. The foundation, drywall, and siding costs were the biggest surprises.

That does seem high given what you are planning to do yourself.

Maybe do a bit more yourself?

You say “tract home grade stuff,” but you are doing a custom edition. You are not buying the quantities that are purchased for tract homes, nor are you getting the best cost in labor.