Stormwater low impact development practices (LID) are lot-level measures which reduce runoff from lots and roads. Although there are many LID planning tools available (ranging from watershed, city, and site levels), it is not easy to find one that is appropriate for development of a LID site plan in Ontario. In particular, the Province of Ontario’s LID performance requirements are based on long-term average annual runoff and pollutant loading reduction. After a review of 16 LID planning tools in North America, it was found that 70% of these tools are site or city level and outputs from these tools include runoff and nutrient loading reduction and cost. However, most tools use average reduction efficiencies based on short-term field measurements or computer simulations to represent average LID performances. As a result, a new LID planning tool which addresses physical constraints, full lifecycle cost, and long-term average annual performance was developed for Ontario, Canada using local precipitation and hydrologic and development properties. The first step is to screen 7 types of (bioretention cells, downspout disconnection, dry wells, green roofs, porous pavement, rainfall harvesting devices, soakaway pits) based on physical constraints and lot characteristics. After the appropriate LID have been identified for a development site, alternatives of LID combinations will be formulated. Using full lifecycle cost (construction, operation, and maintenance costs) and average annual runoff and pollutant loading reduction efficiency (using the US EPA SWMM LID module over 20 years of hourly precipitation for different regions of Ontario), the best LID alternative will be selected based on the achievement of the government performance requirement and the cost-effectiveness considerations. The planning tool was demonstrated using a 79 hectare residential and commercial site in Town of Aurora, Ontario.