Garden offices are a well-established industry in the UK; Shedworking has some tips from Lynn Fotheringham of British garden office builderInsideOut Buildings for making them green. They are points that could apply to any kind of building; some of them cost a little more, but when you are building something small, the incremental cost is not that huge.
Another factor when you build small and well is that there isn't a lot of volume to dilute toxins that get into the air, and it is not likely that you are installing a heat recovery ventilator, so it becomes very important that you use healthy materials, or it might end up like a FEMA formaldehyde trailer.
Some of Lynn's points below, my comments in italics.
Shed interior from InsideOut buildings
1. PVC windows
Problem: People think that double-glazing is eco-friendly because it saves energy, which is true. But look further and you discover that the amount of energy used to manufacture UPVC is enormous. Also PVC frames are bigger and wider than wood and so reduce the amount of light coming through a window.
Solution: Buy locally from a joinery manufacturer and check the wood comes from renewable forests, ideally in Britain or Western Europe. (there are lots of North American sources of wood windows, but you should also check for used windows at Habitat for Humanity and other deconstruction sources)
2. Wooden cladding and flooring
Problem: This is often used on garden buildings and around the house. The assumption that wood = eco-friendly is simplistic. Many cladding and flooring products are pressure treated and imported, both of which use unnecessary energy. So while wood is better than manmade claddings and floorings, it can still be energy-expensive.
Solution: Choose untreated British or European wood from well managed local forests. Our beautiful larch cladding, for instance, is grown in Scotland, then cut and planed by our local timber merchant, with no treatments or preservatives. (in North America there are lots of FSC certified woods available now, as well as resawn boards. Keep away from ANY pressure treated woods.)
Problem: Insulating your home or garden room properly will help the environment, again by saving energy. To do the job properly, you have to use a lot of insulation material. In garden offices, for instance, all surfaces should be insulated to an absolute minimum of 100mm. We use UK manufactured insulation materials, either sheepswool or Rockwool, from firms who have an excellent environmental policy in their factory. Which means we can insulate our buildings’ walls to a generous 200mm for maximum insulation efficiency while remaining environmentally responsible.
Solution: Look at the manufacturing process before choosing an insulation material. Can it be recycled? Has it been imported? Does the manufacturer run their factory processes sustainably? (In North America one can get Denim, but most shed sized buildings are made from 2x4 studs, which will give a maximum of R12 insulation with wools. Again, since the areas are so small, I would consider a soy foam or icynene if I could get an installer to do such a small job.)
7. Paints and wood stains
These are the ingredients to avoid in paints and stains: Solvents and white spirit, turpentine, terpenes, ethereal oils, and acrylics. Ingredients to look for:- Water, binders , non-toxic pigments, mineral fillers and waxes. (absolutely critical in such a small space to use formaldehyde free, non-toxic finishes. I would also not bring any furniture or millwork that was made with formaldehyde binders and would think twice about having a laser printer. You want to start with clean air and keep it that way.)
More at ::Shedworking
More Garden Sheds on TreeHugger:
Friggebod - The Traditional Garden Shed Made Green
Energyspace Sustainable Garden Buildings :
Nature in the Garage