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The Performance Solution Method
If a home extension exceeds 25% of the original home a "Performance Solution Report" might be required, to help reduce cost due to practicality and limitations imposed by the existing dwelling.
Under Reg233 partial compliance can be acceptable to an extension if the floor area of the extension is not greater than 25% of the floor area of the existing building, or if all alterations (including the extension) does not exceed 50% of the existing volume of the original building. Otherwise the entire dwelling needs to meet 6 stars a DTS is possible also (subject to RBS).
The Deemed to satisfy method (DTS) can be used, but due to the difficulty in passing the glazing calculators portion of the DTS , the two reports method might be the more desirable method, now called "the performance solution" in which the following is required:
First report needs to be produced for the original home, which will rate under 6 stars (houses built in 1990 averaged about 1 star on the NatHERS scale).
Second report for the entire new home (existing/alteration and new extension). We will likely recommend upgrading a few key areas of the existing home to bring the whole home up to standard. These are often cost-effective and thermally efficient outcomes. the outcome is generally between 4-5 stars.
A third report, named "Performance Solution" this is to help justify the equation and methodology of the 1st and 2nd report, where the most likely outcome is the new extension will be 6 stars and overall house could be a lower star rating).