The Provisional House is a mandala practice crucial to the Psijic Endeavor

In Sermon 19 of The Thirty-Six Lessons of Vivec, Vivec enters "a non-spatial space" and builds "the Provisional House at the Center of the Secret Door" as a base of operations for his "attack on the eight monsters", where "he could watch the age to come". The sermon concludes with a poem about the things buried in the four corners of the House.

According to The Changed Ones, this was a known practice:

[Boethiah] showed them, with Mephala, the rules of Psijic Endeavor. He taught them how to build Houses, and what items they needed to bury in the Corners.

I think the Provisional House is a dream-mandala, as described by Jung in Individual Dream Symbolism in Relation to Alchemy:

The true mandala is always an inner image, which is gradually built up through (active) imagination, at such times when psychic equilibrium is disturbed or when a thought cannot be found and must be sought for […]

We can see from the mandalas constructed in solid form that it is really the plan of a building. The square also conveys the idea of a house or temple, or of an inner walled-in space. […] the squaring of the circle is a stage on the way to the unconscious, a point of transition leading to a goal lying as yet unformulated beyond it. […] The arrangement of the snakes in the four corners is indicative of an order in the unconscious […]

In the present case the point seems to be to capture and regulate the animal instincts so as to exorcise the danger of falling into unconsciousness.

After items have been buried in the four corners, the Sermon declares "your house is safe now". As with the mandala's four corners, they are protective talismans to safeguard the unconscious and establish boundaries of the Self. The "non-spatial space" is a state of meditative imaginative dreaming, perhaps related to "the god place. The place out of time" that Vivec claims to experience when "completely asleep".

In an IRC discussion, MK gave this description of Sermon 19: "He makes the Provisional House. He attempts the Dream." This explains why Boethiah and Mephala taught the Dunmer how to construct Houses as part of the Psijic Endeavor. The Provisional House is a state wherein a Dreamer may Dream, and more importantly, may be protected from negative influences while doing so.