It looks like the landlord is amenable to having a room built within the barn area of our house.

I've just taken some measurements and it's got the potential to be near perfectly sized based on the Golden Ratio, which is pretty much optimum for minimising the effects of standing waves / room nodes. Looks like it will be around 450cm x 730cm x 260cm in size. I now just need to convince the landlord to build the two extra walls in breeze block rather than plasterboard on a framework.

It's also going to have my office space in it too, and will probably double as a spare bedroom for when we have guests.