EPC's are visual inspections and lets assume that there is a good level of insulation in the loft (for example) but the loft is fully boarded. In the absence of a formal certificate the assessor has no way of knowing how much insulation is actually in the loft and therefore has to enter into the EPC that the insulation is "unknown". We cannot take a home owners word for it as this can lead to legal issues! This is an example of how EPC documents work but for a deeper explanation, please watch the accompanying video.

Nothing has changed in the property but the EPC rating is lower?

There are numerous reasons that this could happen so here are some of them:
1. The property is older since the previous EPC inspection was carried out
2. The backend of the EPC criteria has changed making it harder to achieve the high ratings that the previous EPC had,
3. One of the assessments are incorrect! This may seem obvious when you look at the document
4. The assessor did not have access to all of the areas or was not provided relevant certificates

Above is just some of the reasons that an EPC rating can be lower but the list is not complete and there may well be further reasons for a lower rating.