They are destructive. They will eat plants, and they will dig up plants when they make dust baths in the soil, they will also scratch up the lawn. Not so bad in summer as it grows back, but in winter, things can get nasty when everything turns to mud.
They are good for clearing the soil in a veg patch, therefore good on allotments when rotated with the veg. But, if you want to eat your veg yourself you will have to protect it with chicken mesh.
2 or 3 in that size of area might be bearable.
If area is mostly lawn and shrubs it would cope OK. Though expect things to be pruned to chook height
The more leafy and delicate (annuals and perennials), the more damage they'll inflict.
Mine's a different set up with the girls strictly confined to their part of the garden. They have a fixed pen of approx 18 m2 for 7 chooks. It is a barren dust bath at the moment. They have eaten everything green within reach including most of the green content of the compost bays which happen to be in there with them. Much as I love my chooks, I wouldn't let them lose in my garden, because I love that too!
If OH is a keen gardener, I can understand where she's coming from. But you can still have georgeous garden and chooks, you just have to keep them separate. See pic.
If you do want them to freerange, then to stop them visting neighbours fences would need to be at least 6' I think, and even then unless wings are clipped, a determined escapologist could get over it. Also, you do need to thing about what can get into the garden. Urban foxes are your worst nightmare.