How tall is your GH? Is there space for another shelf above the ones you have planned? The reason I ask is because you also need to have room for fans to circulate the air in there or you may run into mold problems. Also, will you be using heating pads under your seed trays? If so you will need to consider where and how to run electricity to your GH. You may also need grow lights for young seedlings to avoid weak, leggy, floppy plants. So a second shelf above the current ones will give you a way to hang those lights.

My GH is the same size as yours and I have all the shelves along the long walls. The back and front walls are taken up with electrical outlets, spigots, coolers, heaters, vents and fans. I have two rows of shelves constructed of expanded metal. The lower shelves are 30" deep and the upper shelves are 15" deep. I would like to have a row of shelves in the center aisle, but I use my GH for wintering over many plants that spend Summer in the yard, so I need the extra space for those.

I do start a lot of plant trays in early Spring, and the reason for the heat mats and grow lights is the cooler temps and shorter days at that time of year. It's hard to fool a seed into thinking it's time to grow, but it can be done. Those seed starting mats are expensive but I have used regular heating pads that worked just fine. I only have two of them, so I start two trays just until the seedlings are up, then start two more. I found it not feasable to keep the GH warm enough to start seeds while I was trying to maintain other larger plants in there at the same time.

Cheryl

I have about the same set-up you have drawn. I moved the center bench (2x6x10) out (towards the door) about 18" so I can walk all the way around without being trapped on either side.