if its an extention then you can put all the sockets and switches at the existing hights. I think you only have to put them at the new regulation hights if its a new build house.
It might be as well just to double check with your BCO, as I seen to remember Dingbat had trouble with the BCO regarding the DDA heights in a non refurb property.
Unfortunatly, right or wrong the BCO tends to have the final say.
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