For website designers, the biggest challenge that their customers sometimes present them is communication. Whether you want a web design agency to handle custom PHP development, e-commerce development or any other type of web authoring, you'll sometimes find that confusion becomes the order of the day when the project gets started in earnest. You need technical specifications and you need them before you get started on your project. You can give a web designer a site to clone, but that doesn't tell them what the backend uses and it doesn’t tell them what the administrator tools are. The site could have been built with an off-the-shelf CMS like Drupal, Joomla or WordPress or it could be custom. Either way, designers cannot always tell from the front end.
It isn't hard to get technical specifications for a website started. In fact, if you can get the basics going, the web design company can likely help you to flesh the rest of it out. There are some basic elements.
Wireframes are wonderful tools for designers to wok from. They give the designers enough basic information that they can create exactly the interface and functionalities desired. Have your team come up with some basics before the project gets off the ground.
Any complex website usually has some sort of a member section. This means that there are going to be different types of users on the website and that the designers will have to know what each of those users is allowed to do. Go ahead and create some phony users—on paper, if you have to—and write down what they can do at the site. For example, what can a guest do versus what a member can do? What can a member do versus what a moderator can do. User roles are at the heart of good security plans and your designers need to know what your users can do and cannot do on the site.
If you have a data-driven website, you'll want a database scheme drawn up. This really helps the designers to build the resource you want and need which, of course, is their ultimate goal. If you need web development for eCommerce, you probably need PHP development for your database functions. You'll need to help the designers understand what you want those functions to provide your visitors and how the designers need to make that happen for you.
There are some big sites you can use as examples, but it's not enough to say that you want a clone of a different site. Remember that any given site has web design and web development that went into it that likely involved a lot of different technologies. IT might have been built on BigCommerce or it might be built on a WordPress or Joomla CMS theme or template with an eCommerce adding. The designers cannot tell without researching it and, even if they do, studying another site to get an idea of what you want may not help much.
Don't get too caught up on the tech specs. Web designers can work with you to put your creativity into technical terms, but they need to know what you really want your site to do. If you can give them good descriptions of what the site is supposed to do for your customers, they can probably make it happen for you. Just remember that you need to be specific and need to communicate clean milestones toward progress that the web developers can aim for.