When designing a wall unit, there are different considerations that one must take into account before ordering one.
Let's start with dimensions...when measuring out your future wall unit, one must not only take into account the physical size of the wall unit cabinets, but also the additional real estate consumed by any decorative accents that sit on top and around the actual wall unit, such as crown moulding, as it can add a few additional inches to all sides of the structure. Don't forget to also consider the size of additional furniture moulding, wood trim, decorative accents as well that you plan to add to your dimension drawings.
Next, remember to account for utilities and accessibility. For example, if you would like a wall unit with potlights/accent lighting, you then need to ensure that you budget for the incremental costs of having your contractor/electrican prepare the area for your electric needs beforehand. You also need to consider your power outlet needs and accessibility to these locations as well.
Now, depending on your TV, stereo and electronics needs, you will also need to consider how to implement hidden cabling for your TV, electronics, cable box, satellite box, speakers, Bluray/DVD players.
Permenant shelving vs adjustable shelving is also somethere to consider as your needs may change over time, and the flexibility of adjustable shelves allows you to modify as necessary so that you can ensure you enjoy your custom wall unit for a very long time!