I haven't asked to hunt a person's yard in years. Not that I'm afraid to, it's just I got plenty of other places to go. Usually I find enough old houses being torn down here and there to keep my belly full on yard hunting. The rest of my hunting is in the woods, old parks, or at the beach, so I'm not too pressed to ask for permission to hunt yards. They seem to come my way enough by torn down houses, or just word of mouth where one of my hunting buddies or I secures a friends house to hunt or something.
I figure years from now when my usual spots are really burnt out I'll start doing more random asking to hunt yards. I have hunted yards recently, but usually it's when a person sees me hunting in a park and we strike up a conversation. I like to say "My favorite spots to hunt are yards if they house was made before 1965." I figure if they are going to offer then that's the moment they will, and in the past I've got a few hunts that way. Last year I was hunting a school building that is sitting on old fair grounds land that took place back in the 1800's for years. The school wasn't that old but I knew through research there was potential for old coins due to the prior fair for years on that land. Anyway, I was hunting along a fence that separated the school from a house. Struck up a conversation with the home owner who came out to watch me hunt. Showed him a silver dime I dug. He thought that was cool and so then I asked...Would you mind me hunting your yard? I only ask if they seem very receptive to the idea of detecting. That's how I gage people. He said sure go ahead.
Now, what I do though is I then make sure to tell them how I dig a plug, what size, and how I put the plug back. And that I try to use a screwdriver instead of digging plugs whenever I can (which I always try to use in yards for sure). I make darn sure they know what they are in for, because I don't want them being offended when they start seeing me dig plugs in their yard. I feel that's important to make them well aware of, because I've heard of people being kicked out of yards when the home owner saw them dig their first plug. If you tell them what they are in for then I think they might not care, but some people will get very mad if they feel you didn't explain things to them and are trying to take advantage....Or to "take privileges" without asking is the way I think about it.
All that said, I won't ever hunt a yard in hot/dry weather. You're just asking for complaints if the plugs die and they end up with brown spots all over their yard. I know of one guy who was forced to re-seed all those spots in a yard and show up every few days to water them. That's what I worry about most, so if I do hunt a yard it's when it's spring or fall. Only time I'll hunt a yard during the summer is when I know the person just doesn't care about their lawn. If they have weeds and don't manicure their lawn, if it looks like the flower garden isn't kept up with, then those are the kinds of yards where people just don't care.
If you plan to hunt a yard during hot or dry times and that yard looks pristinely taken care of then IMO you are asking for a lawsuit if they find brown spots all over their yard a week or two later. Just not worth the hassle to me, so if I feel they are the "uppity type" I won't risk hunting their yard. Or, I'll ONLY use a screwdriver and not dig any plugs. And a screwdriver should be used as much as possible even if you don't feel any risk with upsetting somebody by digging. Also a good idea to use a screwdriver in parks and other public places as much as possible. Better image for the hobby, and makes for quick recoveries when you get some practice at it. I like to use my Pro Pointer super tuned to pin point the target and then pop it right out. Works great.