To “add to the building”, in this context, means to upgrade or build additional features onto the house. By “discreetly tely”, I think they mean by causing as little disruption as possible. Perhaps to avoid offending the neighbours. Perhaps, though I believe less likely, to avoid causing offence. The building was described as quite large and so it could be seen as an example of boasting or “showing off” if the owners openly spent a lot of money on their already expensive property!