Most environmental law is EU-derived, and implemented via UK law. Technically, therefore, Brexit would make no difference to the applicability of these laws to UK business. That is less true of planning law, which is largely, though by no means entirely, UK-based. That said, for both political and practical reasons, the UK Government is likely to be under pressure to, at least, simplify some of the current rules. Even some Remainers (including George Osborne) have criticised EU environment and planning rules, for instance, describing rules on the protection of habitats as placing a "ridiculous" cost on business.
However, there are two factors that would restrict the UK's ability to initiate a major retreat across the board in environmental and planning laws: first, any deal with the EU which involved continued access for the UK to the single market will mean that EU rules on product safety and waste (amongst others) would continue to apply to businesses exporting to the EU and secondly, the UK's international treaty obligations on public participation in environmental decisions and access to environmental information would continue to apply to the UK, thus meaning rules on environmental impact assessment in particular will survive in some form.
The extent and manner of the UK's participation in the EU ETS will be an area of some uncertainty, however given the UK's historical leadership in this area, and its signing of the 2015 Paris Agreement, a substantial watering down of emissions-related commitments is highly unlikely. Depending on these matters, we might see some softening in the UK environmental and planning rules through the emergence of mirror systems to those of the EU but a wholesale retreat from environmental and planning protections seems unlikely.