Balcombe has signed a new four-year contract.
The 23-year-old is a local lad who has been with the club since the age of eight.
He has now extended his stay until 2027, with the club having the option to extend it by a further year.
Balcombe, who made his senior debut in August for the club in the Carabao Cup, wants to push on to becoming a regular.
“I'm so pleased that he got his deserved debut against Newport and had an unbelievable impact in the penalty shootout.
“No player's pathway is the same. Ellery has been on a natural path as a goalkeeper: going on loans, developing and returning to us. You can see how hard he has worked and how much he has put in.
“He has been more consistent, and he has big potential. We're pleased that he sees his future here because we definitely feel the same way."