Children spend much of their time at home, indoors and sedentary. This study reports on the development, exploratory factor analysis, validity and reliability of the HomeSPACE Instrument. The instrument assesses features of the home physical environment that influence children’s sedentary behaviour and physical activity, and the family influences that create this environment. The space and equipment audit achieved good to excellent criterion validity and test-retest reliability for equipment, outdoor features and home design measures (Study 1, n = 36 parents). Family influence scales showed acceptable internal consistency and test-retest reliability (Study 2, n = 96 parents). Factor analysis highlighted fifteen scales to assess the importance, preferences and supportiveness of the home environment for activity. The HomeSPACE Instrument extends previous tools to provide a valid and reliable assessment of home influences on children’s sedentary behaviour and physical activity, that is adaptable for varying home physical environments.