You want small...I'll give you small. The G-box by Sarah Wigglesworth Architects. This tiny little space is a plywood-clad cabin design and comes fully insulated so that it can be used all year round. It is prewired and comes ready to be hooked up to the electrical grid. The website says it comes in a kit and can be fully assembled by two people. If you live in London, you can have one of these things delivered, by crane, completely assembled. If you had the right set up and could smoothly integrate the G-box to your home (not necessarily attaching it but setting it up in the backyard or something), you could definitely add additional equity to the market value of your home without a lot of costs incurred for construction, drawings and renderings from architects, etc.
There seems to be some flexibility in the type of windows and door you can install on the G-box. Not only type, but where you place them physically on the structure is up to you. Take advantage of sunlight, etc. The G-box is the perfect size for a home office, yoga room, playroom, blogger's hideout, etc. I think one of these things would go perfect in my backyard...problem is getting it here to the states, probably some shipping costs involved. You can even see a rainwater outlet poking out from one of the windows. That's cool! SW promises a G-box website pretty soon! I'll keep you posted on that and on prices...if I find some. BTW, I just realized its Friday the 13th...be careful out there!