Hi Tracyann, and welcome to The Tortoise Table! I'm so sorry that we're a bit late in responding to your post, but we were having a bit of work done on the Forum and website over the weekend and we've only just got notification of posts from Sunday and Monday.
It's a lovely little Horsfield you've got, and it is great that you want to give him more space! Tortoises do need much more space than people think, and in fact they get bored and a bit lethargic if they are confined to a small area.
Converting a greenhouse is a great idea, and many people do use greenhouses for their tortoises, usually with an entrance/exit where he can come and go at will. They manage this by standing the edges of the greenhouse on bricks or breezeblocks and leaving one block out and putting one of those plaxtic flaps there, and the tortoise can go in and out as he pleases. I can only see a few problems or considerations:
1. In the summer (lol, even in Scotland), it can get super hot in greenhouses and tortoises confined to them have been known to die from dehydration fairly quickly. My little greenhouse last summer got to over 45C on several occasions when it was in the high 20's outside. My greenhouse has automaticw windows that open when the temperature gets warm, and it is a good idea to have these (you can buy them separately and install them and they aren't electric or anything -- just solar powered or contain a wax in a tube that expands when it gets warm and makes the window open. Just google it and you'll see lots (
https://tinyurl.com/1b4y6ghb ). Even with these though my greenhouse got too hot for a tortoise, so if you had a means whereby he could go out of the greenhouse into a secure little section of the garden that would be perfect.
2. Remember that UVB doesn't pass through glass, so he would either need access to the outdoors and/or a UVB light in the greenhouse.
3. Cold. Does he hibernate, and if so, where? If he is staying out there all year round, how will you ensure that it remains warm enough in the greenhouse for him? Heat escapes quickly through glass. You could insultate most of it with bubble wrap, but I think that in cold weather when he isn't hibernating it could be quite expensive to keep the temperature warm enough for him (you want one area of around 30C and other areas around 20C).
Many people do let their tortoses hibernate in the soil in greenhouses, but just remember that Horsfields are a burowing species and if he digs down into the soil there is a chance that he could dig down, under and out.
Having said all that, I do think you might be able to make it work, but just thought I'd point out considerations. Also, I wasn't sure what your second photo showed -- the photo of the room with wooden floor. Sorry if I'm misunderstanding, but hope some of that is of use.
Nina