The block quotes below provide a good explanation of what capital "A" Architecture means, and why we like to move freely between the minuscule and the majuscule, to the point where the difference becomes fuzzy....courtesy of Yvonne Gaudelius' essay "Kitchenless Houses and Homes, Charlotte Perkins Gilman and the Reform of Architectural Space"
..."there is a great deal of difference between Architecture with a capital "A" and architecture with a small "a". Big "A" Architecture is Art (also with a capital "A"), whose existence and health rely upon a profession that cannot afford to have itself associated with the blue-collar activity of building. Architectural critic Diane Ghirado points out in her introduction to Out of Site: A Social Criticism of Architecture that “there is a tacit and often explicit professional agreement that nonarchitect-designed buildings cannot be considered Architecture”. That big "A" Architecture is kept separate from little "a" architecture is a matter of professional survival for architects.
Little "a" architecture, however, comprises approximately 80 percent of structures that are built in the United States. Just as literary canons are formed around the exclusion of, for example, the works of women writers, so too architectural canons are established. Not only do these canons exclude small "a" forms of architecture, but they also serve to keep in place the relationship between architecture and ideologies such as gender. Through the virtue of the power of art, big "A" architecture legitimates what is built. A feminist analysis of architecture, by placing gender at the center of inquiry, necessitates a dialogue that unveils this paradigm. We need to move away from typical formal dialogue that establishes dualisms and dichotomies within architecture - for example the distinctions between big "A" and little "a" architecture, public and private domains, interiors and exteriors, form and decoration, which are conditions in which one term, by necessity , becomes subordinate to the other."
From: Charlotte Perkins Gilman, Optimistic Reformer, Jill Rudd and Val Gough, editors. University of Iowa Press, 1999