The touch-me-not cottage is the house that the Foster family lives in. It is called the touch-me-not cottage because everything appears so incredibly neat and ordered that to touch anything might mess up the perfection. That's the main reason why people actively avoid the Foster cottage. The house is uninviting because of its perfection.
On the left stood the first house, a square and solid cottage with a touch-me-not appearance, surrounded by grass cut painfully to the quick and enclosed by a capable iron fence some four feet high which clearly said, "Move on—we don't want you here."
Winnie herself feels stifled in her own home. It's why she is contemplating running away. Later, when she finally arrives at the Tuck household, she is amazed as the disarray of everything. She is not disgusted or appalled at the Tuck household though. On the contrary, she is amazed at how welcoming it feels. It feels like a family lives there, loves living there, and loves living there together.
The "touch-me-not" motif has been borrowed and used in other books and movies too. In Gary Schmidt's book The Wednesday Wars, Holling Hoodhood lives in "the perfect house." Everything is so neatly ordered and placed for the proper appearance that Holling is not allowed to play and be a kid in his own home. It's not an inviting place. The film Ferris Bueller's Day Off does the same thing with Cameron's house. Here is what Ferris has to say about Cameron's house.
"The place is like a museum. It's very beautiful and very cold, and you're not allowed to touch anything."
In all of those cases, the "touch-me-not" concept is meant to highlight a very unwelcoming aura about the location. In each case, a perfect looking house doesn't always indicate a perfect home and family on the inside.