First of all, a new build is VAT exempted, thus is 20% cheaper than an extension.
So, if you have a house and want something bigger maybe this option can be considered.
An advantage for a new house is better thermal efficiency plus a new configuration which can be more desirable.
If you can't decide or don't know what's best suitable for your particular plot book a free consultation with an architect with no obligation on your behalf.
Gym, sauna, Jacuzzi and sitting area.