 The HDB also approves renovation contractor registrations to enforce quality control.Large-scale improvement works to existing public housing developments were carried out in the form of various programmes under the Estate Renewal Strategy, beginning with the Main Upgrading Programme (MUP) since 1990.These help to bring common facilities up to standards with newer developments, and in some cases, to offer some improvements to individual units, such as the addition of reinforced bomb shelters which can double-up as an additional room during non-emergency periods.