Mr and Mrs Moore own a main residence together. They decide to purchase an additional dwelling for their son to live in whilst he is at university. At the end of the day that is the effective date of the transaction, Mr and Mrs Moore own more than one dwelling and are not replacing their main residence, so the ADS will apply. It does not matter that the additional dwelling is not being rented out and is kept for family use only.
If, however, Mr and Mrs Moore were to give their son money to purchase the dwelling and he was to take title in his own name then, provided their son did not own another dwelling, ADS would not apply.
If their son took title to the dwelling and was under 16, both Mr and Mrs Moore and their son would be treated as one economic unit therefore ADS would apply as Mr and Mrs Moore already own another dwelling.