It depends on the distinctiveness on the design. Some designs can be exclusively owned - meaning you can't use it. Some designs (or design concepts) cannot be exclusively owned - therefore you can use your own version of it.
You can't just use an existing design without permission. You need to contact the designer and ask about licensing the design.
If you want to create your own version of the design, the legality is based on distinctiveness (as mentioned above).
Generally speaking, if you are creating your own version of an existing design, you would have to make more than just a few subtle changes to make it legal to use.
If you create your own original design, you can submit it for copyright registration. You will then own the design. Similar to using someone else's design, exclusivity will be based on how unique the design is.