Is DesignBuilder approved by CIBSE for plant sizing applications?

There is a common industry misunderstanding that CIBSE approves software for heating and cooling loss/load calculations. That is not the case. Although CIBSE state that they do not endorse any particular HVAC design or analysis software tools, they do state that dynamic thermal modelling software such as DesignBuilder is suitable for both local and central HVAC plant sizing. See CIBSE Guide A, Figure 5.2, Section 5, for more details. Also, further discussion on the topic can be found in CIBSE Guide A, Section Appendix 5.A1: Overview of calculation methods.

DesignBuilder EnergyPlus uses the ASHRAE Heat Balance method for heating and cooling sizing calculations, which is generally regarded as more accurate than the CIBSE Admittance method for system sizing applications.

In summary, the ASHRAE Heat Balance method used by DesignBuilder is an acceptable method according to CIBSE Guide A.