pinktwink, there is no law to say you cant leave a 12 year old alone at home, at this age most are absolutely fine to be left for a few hours, it all depends on the individual child and it does not make you a bad or neglectful parent if you are leaving them at this age or even a bit younger. I was left alone for a couple of hours after school when i was 10yrs old and all was fine. I am so grateful to my parents for giving me love and security as well as freedom and a bit of independence.
I dont think it makes any difference if its now or 20 years ago.
It sounds like you are a very caring parent who wants to give your daughter some independence, i believe this is essential for children, too many of them now are overprotected and it does them no good for when they are adults.