The average size residential project in the study was 19,600 sq.ft., with a landscape area of 15,680 sq.ft. (80%). The total average costs per square foot was $4.34 to $17.83 for contractor installed projects and $4.94 to $19.53 for professionally designed and coordinated and contractor installed projects.The average costs for a do-it-yourself project in the study was $2.20 to $9.77 per sq.ft.he total average costs per square foot was $4.34 to $17.83 for contractor installed projects and $4.94 to $19.53 for professionally designed and coordinated and contractor installed projects. The average costs for a do-it-yourself project in the study was $2.20 to $9.77 per sq.ft.