As with new build houses there will be a build-up of detrimental emotional energy patterns, left by the tradespeople that will need clearing away.
What was the building before the conversion was carried out? A hospital, psychiatric/mental health unit, chapel, mill and so on will all leave emotional scars or imprints on the walls, floors, ceilings etc. That in turn can cause both mental and physical health issues to the unwary new occupants.
Converted chapels and churches are normally deconsecrated, but that is not always the case, this is something of which to be aware and the question does need to be asked of your solicitor.
An extension to a house is just as the word implies – an extension. It will remain that way unless it is energetically brought within the outline of your house.