Delivering the UK’s greenest prison to date

HMP Millsike is a Category C resettlement prison for the Ministry of Justice in York, being constructed near to the site of the former Full Sutton prison with a public main contract value of £400 million. The new site provides close to 1,500 additional prisoner places to HM Prison and Probation Service (HMPPS) to meet the growing demand for additional capacity across prison estates.  

The UK Government has committed to a 10-year strategy for increasing prison capacity, unveiled by the Lord Chancellor in 2024. Mace has worked with the MoJ since 2019 as Delivery Partner for three prisons opened as part of the ‘New Prisons Programme’ (NPP); HMP Five Wells in 2022, HMP Fosse Way in 2023 and HMP Millsike in 2025.   

This is the first prison to be delivered through the Mace-led Alliance 4 New Prisons initiative. Having led delivery across the entirety of the NPP, Mace has continuously shared lessons to benefit future sites, whether through incremental design improvements or opportunities for future innovation. 

At HMP Millsike, the team have delivered the UK’s greenest prison to date. It is the first fully electric prison in the country, achieved through the implementation of Air Source Heat Pumps and PV panels to supplement power drawn from the grid which significantly reduces carbon emissions. In addition, throughout construction we have focused leaving a positive environmental legacy while minimising our waste. This includes a 10% increase in biodiversity net gain and a BREEAM rating of 87.1%. 

Construction of the prison began in 2021 and completed in 2025. The handover from construction to the operational team was heralded as a great success ahead of MoJ’s key first-prisoner-in date, demonstrating Mace’s commitment to driving continuous improvements across the NPP.