Knock-off legal?

I think if you copy something for mass production re-sale you have to
change or modify it by 30% to make it legal (from what I recall in my
baby swimwear production days).
But, if you are just making something for yourself can you copy
something you have seen from a major designer to the best of your
ability?

What are the legalities of taking pictures in the dressing room?

Ok, ok......I have a confession. I should have never done it. I knew
it was a bad idea from the start. But, I went with a friend, just to
make a return to the "special occassions" department and have lunch at
the department store. A dress caught my eye....just a glimpse from
the corner. I shouldn't have turned to give it a second look, but
oooh when I did I fell in love. In love with a $330 dress! A dress
that seconds later, before my friend could put her return money back
in her purse, I was swanking around the dressing room in. It was bias
cut and look perfectly wonderful - with no bra mind you!
One bad idea quickly follows another and soon I had my husband at the
store....just to show him. Because I havn't spent $330 on all the
dresses I have ever owned much less on just one that I would have
absolutely no place to wear anyway (except my friend wanted me to get
it so badly that she immediately invited my husband and self to a
black tie affair). But, the dress was sold. I was just looking at it
anyway. Enjoying the fantasy of windowshopping to the max.
But, now I am visiting the dress at the designers website.

Something is terribly wrong with me and I must get that dress!
But, in a legally approved seamstress sort of a way.
So, in copying ready-made garments, what's legal and what's not?
Joy