So do you put them out for grass during the day and then bring them in to sleep during the summer or do yours have a separate outdoor hutch where they sleep for the summer months?
They should only be out on the grass if it's dry and reasonably warm, and at first you should gradually introduce the grass to them so they don't get upset tummies. If it's not warm enough you can still hand pick grass for them, though you have to be sure the grass has not been fertilised or has been sprayed with pesticides, no animals have defecated on it (cats, dogs, foxes, etc.), and if the grass is soaking wet you can use a towel or kitchen roll to make it dry enough for them to eat.
What I do is put the piggies' run outside, wait for it to rain (As this is Britain I don't have to wait for long, haha) so it washes off anything that might be on there, and once it's dry, the run area will most definitely not have any animal waste in it because the rain washed it off.
Remember, as was mentioned in the first post, be wary of weeds that may be poisonous.
If it's warm enough but the grass is too wet, you can put something waterproof on the patio (e.g. correx, shower curtain) and place fleece on the top so they can have exercise. I suppose you could alternatively use loads of towels or kitchen roll to dry the grass, but you would have to be quite thorough.
As for where they go during the night (or any other time they can't go on the lawn), it's up to you whether they stay in a hutch or an indoor cage. Outdoor piggies who have lived outdoors for a while will be able to cope with colder temperatures better than indoor piggies, though it still shouldn't be too cold if they have lawn time. If you want your indoor piggies to move into an outdoor hutch, please wait for the weather to be a little warmer consistently so it's not too much of a temperature difference for them. Once the autumn/winter comes and it gets colder, they can still live outdoors because they are already used to the outdoor weather, but the hutch will need some insulation.