Bigger living area = weeing problems!

I have had a mini lop buck for just over a fortnight. He is 13 months (not yet ready to be neutered). I am off work at the moment so I have had lots of time to train him and I litter trained him in a very short amount of time. He has a litter box in his house and he has designated a place in the living room when he prefers to pee, so he has a litter box there which he prefers over the one in his house (which he uses only at night). I ordered a puppy pen which arrived today so that I can extend his living area at night and when we're not around to supervise him (he can roam free in the house when we're home). At bed time I extended his house by a great amount so he can tazz around in the night if he wants to but as soon as I did this he peed in the new space. I have been down here supervising him and he's done it another 3 times 20 minutes! It's very deliberate as there's only a little wee each time. He sometimes drops pellets when he feels an area needs to be more rabbity and he's doing that much more than usual along with the peeing.

It's so frustrating because his litter habits are usually PERFECT! I think I'm being a super kind mummy, giving him a bigger house to roam in and he just pees all over it! Should I just keep him in his current house at night and forget the extension idea or is there a way to curb this weeing behaviour?? I can only think that he's marking his new space with wee and poo. I have put him in his original house and closed the door so he can't access the 'extension' and he actually seems more contented. Suggestions would be very welcome!

It's probably because you have extended his area so he is marking it as his territory. They are all slightly different when it comes to toilet training so it is hard to give any advice as to what to do.