We do install a lot in new builds but not often via house builder expect one house builder does get commission if they get they can get home owner to get alarm fitted which I think works well however our sales team go and target the new homes after a few weeks of people moving in. Which most people do welcome alarm just to feel safer as if you talk about all there new gadgets and how you don't really know your neighbours and how burglar more likely to rob a house without alarm.
Yea most people do expect Wireless Alarms and I understand that with a new house but again most new houses you can't paint or put much on wall as you need allow the house to settle.
Adding Circuits - We are like other poster, We get our sparks to come in add a circuit and try get any extra jobs while at it from customer but i unsure what actual rule is as I know company policy is I am not allowed to touch electrics on any property and I must call our sparks however our sparks work for same company so I think its just insurance but I can image it be law as you don't want 'Dodgy Alarm Fitter who thinks he knows what he doing and burns down a house'