In Tuck Everlasting, why is the cottage labeled as a "touch-me-not"?

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. 

Why was the Fosters' house called the "touch-me-not cottage" in the story Tuck Everlasting?

The fact that the Fosters' house is called a "touch me not cottage" indicates that the cottage is pristine but not inviting. The area around Treegap, where the cottage is located, has a notable natural beauty, but in the vicinity of the house that beauty is spoiled by the declaration of human ownership - the woods belong to the people. The house itself is

"a square and solid cottage with a touch0me-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.'"

The house is neat, clean, and "proud of itself." Ironically, it pristine look is forbidding; it is clear that the owners exercise strict control over all aspects of life here, and strangers are not welcome.

Winnie is the only child of the Foster clan, and she is very unhappy in the Treegap cottage. Although her family loves her, they allow her little freedom. She finds the atmosphere in the touch-me-not cottage stifling; like the feeling it exudes towards all who pass by, it is not a place where one can act spontaneously and feel at-ease. The Tuck's home, in contrast to the touch-me-not cottage belonging to the Fosters, is messy and unkempt. Winnie, however, discovers that, surprisingly, despite its appearance, the Tuck's home is more welcoming than her own.

Why is the Fosters' house in Tuck Everlasting called the touch-me-not cottage?

At the turn of the century, when Tuck Everlasting is set, there were strict rules governing the behavior of the female sex. Young girls usually wore white, long dresses. Winnie was being raised to be a proper young lady of her time. She was not to play or get dirty. Her family was an old, established one with a reputation to maintain. The cottage where she and her family lived, was kept immaculate. So in that way, it was a touch-me-not cottage. Touching it would mess it up. A forget-me-not is also a kind of flower that probably grew in the gardens. But more than that, Winnie's family was "above" that of their neighbors. They would not want to be touched or polluted by the common folk. So, all three of these elements could fit the title of the cottage.