Within the government's Biodiversity Metric, "Enhancement" refers to taking an existing habitat and improving its condition, or taking an existing habitat and "upgrading" it into a higher-rated but related type of habitat. In both cases, you are retaining what is there, and improving it.

Government guidance states that you should "ensure [you] have the expertise available to achieve the desired intervention within the required timescales."

Any enhancements which provide meaningful net gain to your development are called "significant" and you have to agree to a legal mechanism that requires you to maintain the enhancement for at least 30 years. This mechanism can either be a Section 106 order, or a new instrument called a "Conservation Covenant". In either case, you will have to set out how you intend to enhance the habitat, and how you will ensure you can maintain it over the 30 year period.