Brand talks about the six design considerations in architecture. The site is the place and foundation on which a building is constructed. In language testing, the parallel is the model of language ability upon which the test is based. These change little over time, but some plate movement does take place occasionally. The structure is the test purpose, constructs of interest, and discourse domains relevant to the population for whom the test is to be built. The testing equivalent of the skin is the delivery model and presentation model, which tell us how the test is delivered to the test takers, and what it will look like. The space plan tells us what we are going to get in the test, in what order, how much of it there will be, and how it will fit into the time available. The "stuff" is the test tasks and items that we fit into the space plan. Finally, the services of a building give it usability. In language testing this is the validation argument that states what evidence and theoretical rationales make the test usable for its intended purposes. As tests evolve we can change elements in each of these design layers. But as you can imagine, changing the site is extremely difficult. It often means starting again, but as in architecture some extensions are possible. Similarly, changes to structure are quite rare, but can and do take place. The other levels, to various degrees, can be changed more frequently. A test can be made longer (space plan). A computerized version can be developed (a change in skin). New item types may be developed in order to better represent a construct (a change in stuff).