A If life were simple, you'd wait to tell her until after your offer on a house was accepted and a moving date was set, to give her enough time to get used to the idea but avoid months of indecision and anxiety. But it's almost inevitable that you'll have to take your daughter along on a house tour or two. And she's sure to hear the two of you discussing real estate at the dinner table.
The fact is, there's no perfect time to tell a child that she and all her possessions will be uprooted and replanted in a new location. No matter when you tell her, she probably won't like it at first, unless you're buying the house next door to her best friend. When our family moved last year, my husband and I decided to tell our then 9- and 6-year-old early on that we were house hunting. Confiding in our girls had its pluses and minuses. It's hard to concentrate on the house you're looking at with a child in tow who likes to touch other people's breakable objects. Junior house hunters tend to fall in love with a home (and beg you to buy it) for completely irrational reasons, such as a nice kitty lives there or the basement has white shag carpeting. We learned to take the girls to see a house only after we'd given it the once-over and knew what to expect.
On the other hand, letting them in on the process gave them the chance to see, over a period of months, that there were lots of other houses besides their own that they could imagine living in happily. And discussing the move gave us the opportunity to emphasize that it's the people in a house, not the house itself or furniture and stuff inside it, who make it a home.