Complex sentence: Nunn Mills is currently under development to build over 3,000 new houses on brownfield site, as well as this there will be a improved Marina in and around Becket's Park and Midsummer Meadow.
Split into simpler sentences: Nunn Mills is currently under development to build over 3,000 new houses on a brownfield site. There will also be a improved Marina in and around Becket's Park and Midsummer Meadow.