Great work publishing a simple tool for people to use. Being pretty basic, it is simple enough for people to use easily and probably good enough for most small projects. I would point out however, that it is really a carbon calculator, not a pollution or environmental impact calculator. It only estimates carbon emissions associated with construction activities (which is a great start). It does not quantify other environmental impacts. You would need to move towards more of a full lifecycle analysis (LCA) methodology to achieve that. Most LCA methodologies incorporate estimation of 11 other environmental impacts beyond carbon emissions (e.g. ozone depletion, resource depletion, human and eco-toxicity, water use, land use, eutrophication, photochemical smog, soil salinisation). There is a tendency these days to equate environment with carbon and this can be quite misleading.