Natural house is a phrase used in modern astrology to refer to a specific set of correspondences between certain planets, zodiacal signs and houses.
The concept is based on the notion that there is a natural correspondence between each of the twelve signs of the zodiac and each of the twelve houses. The first sign of the zodiac, Aries, is “naturally” thought to correspond to the 1st house, by virtue of the fact that both come first in order. Taurus, the second sign of the zodiac, is thought to correspond with the 2nd house, by virtue of both being second. Gemini is thought to correspond with the third house, and so on.
The concept of associating each of the twelve signs of the zodiac with each of the twelve houses is then extended to the planets as well, through the rulerships of the planets over each of the zodiacal signs. So, because of the association between Aries and the 1st house, the planet Mars is then also thought to have some natural association with the 1st house as well, due to its rulership of Aries. Venus is thought to be associated with the second house due to the connection with Taurus. Mercury with the third house, and so on.
In this way the phrase “natural house” can be applied to both the signs of the zodiac as well as the planets, since both the signs and the planets are thought to be “naturally” associated with a specific house.
The concept of associating each of the 12 signs of the zodiac with each of the 12 houses in this way seems to have originated during the Medieval tradition, although it wasn’t until the 20th century that the doctrine was expanded in order to become a major tool for determining the meanings of the signs of the zodiac and houses. By the late 20th century it became one of the core principles underlying the rationale for assigning meaning to the signs and houses.
Earlier in the tradition this does not appear to have been the case, but instead the significations of the houses seem to have been derived largely independently of the signs of the zodiac, in accordance with other considerations.
Houlding, Deborah, The Houses: Temples of the Sky, Ascella Publications, London, 1998 (2nd ed. rev. 2006, Wessex Astrologer, Bournemouth).
Oken, Alan, Rulers of the Horoscope, Crossing Press, Freedom, CA, 2000.