Good luck, I've done one about 20 years ago and it was a real pain in the you know what. Don't forget to use pi when determining the square footage. After that, I got out of residential and stuck to commercial.
You may have to do some digging and go outside of the norm to get this done. You may have to go back sometime as well. Are there other Domes in your area?
I've done three 8 sided houses one of which I did twice.
No similar comps (of course). What you have to do is find houses with similar site and size/utility. Then you have to figure out if this design is a plus of minus in the market. Check any old sales of the property or any similar anywhere and see if you can measure a market reaction to the design. Don't automatically think it will be a negative though if the market really loved the design, you could surmise that there would be a lot more of them.
The only thing you can do is to work it through systematically and explain everything.
I do one about every ten to twelve years and I did the last one about six years ago so I'm off the hook for another four to six years.
Even though I feel your pain not much to offer except that you explain your logic in your report. There probably aren't any comps so no one will throw many or large rocks at you as long as the logic is reasonable.
There are a couple of threads of ones I have done here in Wisconsin in about the last 18 to 24 months. I posted pictures of both of them as well.
I used comparables from similar rural markets across the state. I think I ended up with three or four. Matter of fact I just seen on the other day that I had no clue it was sitting there. Been up and down that county road for the last 15 years or so and never notice it before.
They can be a pain in the butt to do at times. The ones I did had upper levels as well. One even had a basement.
thanks for your responses so far. the diameter is 39.8feet which comes to about 1250sf on one level. The upstairs has about 1/4 of it open to below. I'm calculating about 2100sf total but the house seems so small inside. It has 3 bedrooms and 2 full baths upstairs. It has a kitchen in the center, a family room, living and dining room and full bath on the main level. It seems like a beat up cape COD would be a good comp as the utility is not that great in the dome. It seems chopped up. It is all covered in shingles.
The area I am located is not rural at all but in the baltimore/washington corridor so comps in the country side would not appear to be viable. I think I will try to stick to homes of the same time period (1977) with smaller rooms and smaller baths and desirable lots. Subject is on a 30,000sf lot at the end of a dead end street.
I'm concerned about the sf calculations as 2100sf seems too large despite the diameter and the space upstairs.