Here, you are expected to have an automatic door closer on the door into the house from the garage. There is a higher risk of fire starting in a garage than a dwelling, or at least it is perceived that way by many insurance companies. The door closer is also intended to deal with CO, every winter there are poisonings or even fatalities from cars left running in attached garages - in most of these cases, the vehicles were accidentally left running.
I could live with additional construction cost for attached, as that is only a one time charge. Depending where you are, the annual insurance $ delta can be high for an attached garage. But, they are convenient in nasty weather, for sure.
I built a detached here, partly for insurance reasons, partly because I wanted to be able to build an addition to the house in the logical spot to attach a garage. Don't plan on attaching the garage, if/when I get the addition built.
Rgds, D.