In order to construct a plan drawing test for emotionally disturbed children, 1,250 children from 6 to 15 years old were asked to draw plans of their houses. Forty items in this cross-sectional sample illustrate a regular and rapid increase with age from the purely vertical pictures of houses at 6 years to the mainly horizontal representations at 13 years. This development took place in four stages: (1) Stage I--when children passed from drawing a vertical picture of the exterior of the house to drawing just the ground floor of the interior; (2) Stage II--when children drew the rooms horizontally and generalized this solution of the problem to other parallelepipeds; (3) Stage III--when unidentified interior space was eliminated; and (4) Stage IV--when the concept of proportion was incorporated. An ability at Stage II to draw rooms horizontally somewhat corresponds to an understanding of vertical and horizontal space. Results with 20 boys and 20 girls (aged 8-9 years) concerning the Piagetian tasks of verticality, horizontality, and Euclidean space show that the associations between the tasks and drawing a plan horizontally were higher for girls than boys. Appendices present: the scoring system with reproduced drawings; graphs showing the percentages of students passing each item; a table listing the order of ages at which 50% of the children successfully passed; and illustrations of typical plans of the four stages from ages 7 through 13 years. (SLD)