As summer gives way to fall, perhaps you remember the joy of the occasional school day when an indulgent teacher would agree to hold class outside. Well, what if you could work outside—every day?

Just eight miles from the center of Madrid, the Spanish architectural firm SelgasCano, a partnership of the husband and wife team José Selgas and Lucía Cano, has created an office in the woods for doing just that.

There's...