[glend (Glen)]

Ok here is what i did. My wall height is 1500mm, and i used a skillion roof with the high end on the pier end for scope clearance. My interior floor dimensions are 1800 x 2400mm and this provides enough room for my concrete pier mounted CGX and swing room for all of my scopes, plus i have a small workstation on the rear wall and can often stop the roof roll back to retain a roof section over the workstation. The height of my skillion roof peak was determined by the closed roof clearance required for my 10" f5 imaging newt, when opening the roof it is easy to release the RA clutch and swing the OTA out of the way.
Your wall height must be determined by the clearance you require to access the lowest altitude you would normally work from, and this can also be affected by trees in other yards, etc. As an imager, i don't work below 40 degrees, and this is enough to clear any trees on all but one side. Mock it up, or make a drawing. I personally think 1800 high walls are too high and you can easily duck your head to get through a 1500mm door opening. Build your pier footing first, as that determines every other placement. My pier is on continous pour of concrete from bottom of the footing to top of the pier and the cage bolt inserts, obviously with reo as required. Your pier height above your desired floor level is an important measurement if doing it this way. The pier extends up through (but does not touch) the obs floor, must be isolated.
My obs is build on treated pine stumps, this provides ventilation under the floor, reduces condensation due to cold slab effects, and prevents torrential rain run off from coming in the uphill door joint. My uphill step up into the obs is only about 200mm, which provides good bearer clearance and an air gap. My bearers are through bolted to the posts, flooring is tongue and groove ply structure floor which i sealed top and bottom before install. Make sure you seal any end grain. Equipment reaches ambient temperatures quickly. As my obs is on a slope, I also ran a trench around the obs and put sleeved ag pipe in for drainage to two pits below the obs. This keeps the ground dry around the footings and prevents water retention under the obs. I used 8 treated pine posts ( suitable for footings) to build on, each sunk to 600mm and concreted in place, with gravel drainage layer at the bottom of each hole.
Prior to putting flooring down i installed a double layer of weed mat on the ground and covered it with gravel, this prevents any growth below the obs and discourages varmits.

My obs is designed to be modular, ie each wall is an assembly, as is the roof and floor. I can break it down to move it if i ever have to, and it would all stack on a large box trailer for transport.

My obs has survived some pretty extreme weather events, including the big east coast low storm which hit it with heqvyvrain and sustained winds of over 100kph. Obviously good roof tie down design becomes important, as well as wall bracing and cross bracing at the corners.

My build thread is still in the Obs forum section if you want to check the photos. Have fun,

But, its your obs so make it the way you want.