I venture that you would like the space to be somewhat cooler in the summer and warmer in the winter?

Yes! The current greenhouse styled room is 8x20, and we will keep this same footprint. But, we will be changing the 45 degree wall of windows to a perpendicular wall, tehrefore we will need to put a roof over this 8x20 area. We are planning to do a shed syle roof. Using the Sustainable by design overhang calculator, I am coming up with approx. a 4'6" overhang. Which seems way too big to me, but I am trusting the numbers. We are in Southern Maine. We are special ordering triple pane solar gain windows from Eagle, so we definitely want those well shaded in the summer. One big concern for me is keeping the overhang high enough that it will not be compromising my view of the backyard. So, we were thinking of making the roof with not much of a pitch. I'll stop there. I have a ton of questions, but I'll wait to get feedback first. Thank you in advance!

I've been considering the possibility of a usable "sun room" (15 x 20) for some time myself. The overhang I figured was ~ 4' - 4.5'.

One issue holding me back is the question of how the space can be made to be like the rest of the house in terms of temperature. The roof would most likely be metal standing seam with a slight pitch and be heavily insulated (> R30). All this is driven by the architecture of the existing house. Another consideration would be shading the roof with solar panels mounted on top with a 6" air gap.

After visiting other homes in CA with flat built up roofs + insulation underneath, I'm not sure how much heat gain I would be able to economically eliminate.

In addition, the walls would need to be significantly insulated (> R30) and sealed.

The floor would be interesting in my situation as the space would be over existing concrete slab. Most likely that would reuire a floating floor over insulation with perhaps underfloor heating.

The idea is to avoid adding HVAC for that area while gaining a comfortable and usable space.

At that point though, the space would become an addition and be subject to city planning, inspections and an increase in taxes.