Hi Sandra, here's a link to an excellent blog about building the 6x8 Harbor Freight greenhouse. The owner of the blog did raise the height of the greenhouse, so it may have the help you're looking for.
Building and Improving the 6x8 Harbor Freight Greenhouse

I have the 10x12 Harbor Freight greenhouse. When we built our foundation, we first laid out the metal base parts, and joined them together, and used that as a template to be sure we make the wooden foundation the correct size. I've had people contact me that tried to use the measurements listed in the 10x12 manual to build their wooden base first...and then found out later the measurements in the manual weren't quite right.

In other words, I think it would be safest to use the dimensions of the actual base pieces in your kit, instead of depending on measurements you find on the web, or in a manual. It would be such a shame to do the wooden foundation...and then find the metal base didn't quite fit!

Here's a link to my blog also. Although there's a little difference in the size, you may find info here that is generally helpful. Specifically, be careful about installing the polycarbonate panels by using only the clips included with the kit. Many folks who have only installed the panels as per the instructions have later lost panels in strong winds. It's easy to add a screw or two through the panel into the aluminum frame, and both of the blogs in this post offer suggestions about how to make your greenhouse stronger in winds.
Building our 10x12 Harbor Freight Greenhouse

Best of luck with your build!

Great! In that case I'm pretty sure you've already found these older threads, about joining two HF greenhouses together, but I'll add them below, just in case:Milwdave joined two 6x8 Harbor Freight greenhouses. I don’t
recall seeing posts from him here recently, but here’s a link to his photos:

Greenhouse Construction by Milwdave

And a thread about Funnylady's greenhouses:

Two Harbor Freight 10x12 Greenhouses Joined Together

Rosepedal joined two 10x12 greenhouses together, and there are some
photos at the end of this thread showing the new one being added to the back of
her existing greenhouse:

We're Going Saturday

I hope you will post back here again, and let us know how you're doing. I try to watch for HF questions, but if I've wandered off, you can also send me a message through my Houzz profile, or my blog...and bonk me on the head. ;-)

Hi Sandra. I am just putting the finishing touches on my 10 X 12 HFGH. I printed out Mudhouse's instructions. All 80 pages LOL. I used most of her advice. Very comprehensive. I had her instructions next to the manuf manual. It was very helpful. I would like to just mention a few highlights. Buy extra clips for the panels. I installed at least twice the amount the manual called for. Tape the panels as she mentions in her blog. I siliconed all of my panels in after they were installed but not the bottom. You will need to brace your GH before you even think of installing one panel. I am OCD and went overboard but I am confident I will not have problems. I built a Unistrut structure inside the GH. The corner poles were cemented in. I insulated the metal frame like Mudhouse recommended and then covered it with PT plywood strips that were sealed with Polyurethane. The metal frame was fastened down to a PT frame beforehand. I REALLY took my time. I built it myself which was challenging at times - especially the roof. Mudhouse's blog was invaluable. I really couldn't have done it without her. Well, OK maybe I could have but it would have been scattered all over the neighborhood after the first windstorm. Thank you, Mudhouse.

Thank you Tom, I'm so glad the blog helped! (Although now I feel like I should mail you a printer ink cartridge, after printing out all those pages!) LOL. I hope you enjoy your greenhouse, it sounds super!

Tom,

Thanks for your input. I to have printed her instructions as well. I have no idea what I'm doing, but I am confident I can tackle this task with reading and researching every piece of information I can get and her information is the BEST!!!

I have no carpentry skills at all, but building a base frame doesn't look like it would be too difficult to install. Just need to make sure when I do build my frame, that it is level and square.. as I have read that it would throw me off later in the project.

I have also ordered xtra clips as well to help secure my units in place a little better, as I live along the Texas Gulf Coast area and we are prone to getting bad winds form time to time, not to mention that we do get hit every once in a while with Hurricane force winds as well.

My question is: Can the instruction guide Mudhouse posted be a guide I can use to build my 6x8 units? I have 2 units that I will be placing & attaching back to back and I'm wondering If I can use her guide to help navigate me when I start to install my units, being that their manuals are really not that specific in their detailing instructions.

Maybe you can post pics of yours, It would be nice to see how yours came along. Thank you in advance and for you taking time to read these posts.

The instruction manual that comes with it does have its flaws but it isn't totally useless. I would bet that there are many similarities between the two. Her improvements are what really matters (extra clips, bracing, taping the panels, bracing, building the door(s) and oh yeah did I mention bracing) ? Somebody saw my GH and said how they bought one and just put it up without bracing. It was not a happy ending. Another thing she mentions is screwing the panels in - I bought the screws and rubber/metal washers at Lowe's (HD didn't carry them). The back wall does not look so secure at first but that's when you improvise. I live in MA and have a lot of space on my West so the wind really picks up. I had to have my 6 year old roof on my house replaced due to wind but I had them "hurricane nail" it the second time. I was very worried about the GH but with Mudhouse's advice it went well. I faced my door to the East. It would never survive the wind. The two weak spots on the GH are the door(s) and that back wall but they are both fixable. The jury is still out on the vents. I don't really care how they don't shut 100% but hey, the price was right. I really wish you luck with it. Take your time and have fun. It really is a nice kit once you modify it (a bit). Still cheaper than the other ones you see. I will post pictures soon.