THE POST BELOW IS NOT GOOD ADVICE. PLEASE DOUBLE CHECK EVERY POST BEFORE YOU SPEND MONEY. THIS COULD POTENTIALLY DAMAGE YOUR PROPRTY.
if it is now internal yes you can fill it or you could even knock it down if you put a lintel in!, the cavity is only for external walls to keep out damp and cold your new wall of your extension will take over that job!
Please ingnore the reply above as the old external wall still sits on foundations underground and it is still possible, extension or not, that this wall could transfer damp up the wall to the inside of the property. In fact this can ONLY happen if you fill the cavity without inserting a damp proof course within the cavity, at the same height and overlapping the existing dpcs in the skins either side of the cavity.
Please also note that once you turn an external wall into an internal wall you must provive a cavity tray in the cavity above the height of the extension if it is single story. This stops and moisture in the cavity dropping down into the cavity and potentially soaking the skins on the new internal wall. Once damp is in an internal wall it is extremely expensive operation to stop it.
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