The modelling process for alterations and additions is much more involved than for a new dwelling. You will need to run three separate simulations:

1. Existing: model the existing dwelling, and document all assumptions about the thermal performance of the existing building fabric.
   
   
2. Reference: model the entire existing dwelling and the alterations and additions, with all new work complying with the relevant BASIX Construction and Glazing commitments. You must not change the assumed performance of the existing building fabric.
   
   
3. Proposed: model the entire existing dwelling and the alterations and additions, but with the actual proposed performance for the new building elements. Only change the performance of the parts of the existing building fabric where new work is being done such as installing additional insulation or replacing a window.

This diagram shows what will need to be included in the simulations of the existing, reference and proposed models.

You will need to demonstrate that the alteration or addition does not reduce the overall thermal performance of the dwelling, and that the proposal performs at least as well as if it had been in accordance with the BASIX Construction and Glazing commitments. That is:

• the total loads calculated for the proposed model must be less than or equal to the total loads calculated for the existing model; and
• the total loads calculated for the proposed model must be less than or equal to the total loads calculated for the reference model.

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