Generally, yes you can. But to answer in details, this depends on the degree of alteration or addition. If the volume of alteration is less than 50% of the volume of the existing building or the area of the extension is less than 25% of the existing building, it is very unlikely that you need JV3 modelling to determine compliance for the new works. In these cases, a simple Section J DTS report is proffered. However, if the degree of alteration and extension is more than that, and relevant building surveyor may demand full compliance instead of partial compliance. In these cases, JV3 is your last resort to keep the existing building from the new works or to replace some old windows, instead of all.