When we were looking at house plans for our retirement home, wife just couldn't find anything with 100% satisfaction. She would fine a kitchen she liked but didn't like the rest of the house and on and on it went with thousands of designed home plans.
Finally I just sat down with some large sized paper and started laying it out with what she liked. After I got most of it roughed out in place, I then did a scale drawing of the floor plan.
We then found a local architect that we sat down with our sketch and our thoughts while she semi-finished out the floor plan. We took that and studied it for a few weeks and then decided we wanted it flipped east to west and had a few other redlines on it that we wanted changed. We took that back and had her redo the floor plan to scale with roof line details, window and door layouts etc.
We studied on this for several weeks and decided that we liked it and it would fit into our building site by laying out a string line to the dimensions. We then sat down with our preferred builder (he had just finished building a house for my brother/sister in law and we liked the craftsmanship and price.
We took the floor plan (2308 sq. ft of heated area)with roof line which is all the builder said he would need. We talked out what we wanted as far as specifications of, concrete slab, wall construction, roof, windows, doors, insulation, paint, HVAC size and a lot more. He came back with a price that showed limits for windows, doors, kitchen cabinets, kitchen appliances, light fixtures, flooring etc. We liked the price but had to wait about 8 months for him to start. This worked out ok as we finally sold our present at that time home which we were able to use that money to pay for part of the home. The rest I saved during the 14 months that we were waiting for the build to complete so I was able to pay cash for the home as it was being built which was great because we didn't have to deal with getting a mortgage approved or the home appraised which would likely have needed thousands of dollars more in plan drawings.
We did have one builder change recommendation that worked out very well and no additional cost. We moved the house back to the edge of the prepared/levelled hillside so that the 12' x 52' back porch turned into a deck with a concrete patio underneath it. This give it the appearance from the back that it has a basement structure.
We bought a lot of the materials like light fixtures, ceiling fans, all the flooring including marble tiles and hardwood flooring for less than the allowance but we splurged on the kitchen (had to make the wife happy) and went over budget by about double on that. IIRC we ended up with $189,500 for total cost and we didn't cut any corners on quality for construction or materials.
In addition to the 2308 heated area, we have a 12x52 back porch with lower slab area of the same size, a 10x22 front porch, an oversize attached garage 25 deep x 35 wide with all insulated and finished interior.
I had previously had the same builder build me a 32x52 shop which came in handy after we sold our house and had to rent one for 6 or 8 month because we had to store a bunch of our home goods that wouldn't fit in our rental.
So my suggestion would be to build your shop/barn/whatever you want to call it, first. We could have finished it out with HVAC, kitchen and bedroom as it already had a bathroom if we had been inclined to live there while the house was being built, but I thought it better to use for storage building for my boat, truck and excess home goods which saved me a storage building fee for almost a year.