Can someone allow their sump pump water to flow onto my property?

No, a property owner cannot legally allow their sump pump water to flow onto your property without your permission. This could be considered a form of water diversion, which may lead to property damage or drainage issues. Local laws or regulations often require property owners to manage their drainage in a way that does not negatively impact neighboring properties. If this is happening, you may want to discuss the issue with the property owner or seek legal advice.