Pre-Build Advice.

Hey! First time post so here goes:

I'm aiming to build a greenhouse for tropicals in my backyard. I've been going to conservatories like the Krohn Conservatory in Cincinatti all my life and I often have to be forcibly removed by my friends, so I figured I might try and bring some of it home.
I've decided on polycarbonate glazing for the south face and the roof with a of old windows from various projects (different sizes!) to use as vents on the west-east sides. The north side I intend to keep either wood or cinderblock. I have a 16x10 plot laid out (which also happens to be the maximum size for unpermitted auxiliary structures here).
I live in zone 6b and I want to keep the greenhouse above 55 during the nights, and I'm somewhat worried about heating costs so in my research I also came across pit-greenhouses. So I thought I'd ask a few questions to see if it's worth it and if it's possible:
a) Would there be any significant savings to use a semi-pit greenhouse? I want to do something like 2-4 feet below grade as the floor level and 6-10 feet above (with the roof sloping up to the north).
b) If so would 2 ft below grade be enough? 4 ft. below grade?
c) With a wood frame and polycarbonate glazing on two faces what kind of foundation should I use with a pit? Using cinderblocks would be expensive and well out of my range of experience and time-available. Is there any way to use wood without having it rot away within 5-10 years?
d) We often get several days of rain where I live so while I may be above the water table I might have leakage problems.

If I can't do a pit I'm sure I'll be able to save energy otherwise, it just seems like a nice idea. Thanks in advance!

First what type soil to you have,sandy or clay?It needs to be at the highest point on your property.Does your yard drain well after a big rain?What type of tropicals are you planning to have? Keeping a greenhouse 55* in the winter time is expensive.I have a 20x30x12,using Lexan polycarb,steel frame,lean to greenhouse,north side is against the south side of my home,it is from east to west. Right now the temp is 36* outside,no wind at the moment,which is rare,where the second windiest city in United states,Amarillo Texas,and the temp inside is 52* with no heat at this time,have a 30,000 btu vent free propane with auto thermostat,it is set to come on if temps drop below 50*. I have 3 20inch box fans running all the time,up high circulating the air.If the temps drop to single temps,I have another 30,000 btu propane heater,and if the temps fall below zero,I have a barrel wood stove for a back up source of heat. It cost me around $500 to heat the greenhouse thru the winter,and another $250 in a chord of wood.My wall structure is double walled with a 2inch space between the panels,I have 3 exhaust fans,along with a misting system as the summers are real hot here. I grow bananas,palms,hibiscus,plumerias,and many shade tropicals,and have 50% shade cloth on the roof panels. This is the third winter for this greenhouse,make sure you keep it tightly sealed,to stop the cold air drafts,will save you many dollars.

I assume your flat side is against the south-wall of your house. Do you have a flat roof/flat side and end panels or a curved roof down one side? Have you ever considered installing heat-blanket material, what's used to help heat and retain heat in swimming pools?

Ken

I use a swimming pool heat blanket and my dryer vent to heat my leanto greenhouse, and the coldest it drops seems to be 34. If I run the dryer I can minimize drop but I sure can't keep real tropicals in it Geraniums and pansies and tomato starts do ok (have a heat mat under the tomatoes and a plastic sheet over the cage around them.) I have a 100 gallon black tank with goldfish under mostly glass for a heat sink, plus a lot of clear jars of water that soak up heat and help stabilize temps for a few hours in the evening. It drops to the teens here and if not on the south side of my house or with the blanket over ceiling vents it would be much colder

Yes flat side on south side of house,yes flat roof,which slopes from 12ft to 10ft over a 20ft width.I have a 50% shade cloth on the top as well.I'm considering a solar pool cover,when it is time to change the shade cloth.