Many elements of a home are customized to some degree, such as the space for a drop-in tub deck with a surround included. While some owners prefer to install the tub without any type of surround, you may want to include some type of finish element, such as tile, slate, marble or granite. The customization will depend on the look you want to achieve, as long as you follow basic building codes.
When you are planning the height for your tub deck, you cannot simply build the deck as high as the tub is. Instead, you need to include at least a couple of inches of extra space. Later, when you actually set the tub, this space will be filled with foam, concrete mortar or drywall mud to allow the tub to settle into material that contours to its base, providing support for the weight of a body and the weight of the water.
The depth of a tub is the space from the front of the tub to the back of the wall where the tub sits. Not all tubs are set into a space where the wall terminates, so if you have an open tub that is in the middle of the room you don't have to worry about space. However, as a general rule you should include at least 6 inches of space along both the front and the back of the tub, so that the deck has a full foot of open space beyond the facial dimensions of the tub.
There are two options for the width of the tub. The first is when you are installing a tub deck in a predefined space that has two walls along the sides. In that case, you are limited by how wide the tub deck can be from the head of the tub to the foot of the tub, where your head and feet rest. The second option is when you are building your own frame in an undefined space. In that situation, the same rule applies as with the depth: at least 6 inches on both sides is preferred.
When it comes to building the deck, you can customize the frame largely according to your personal preference. This means that if you want a foot or more of space on the tub deck beyond the rim of the tub, you can include it, covering the space with tile or some other material. As long as you aren't limited by preframed walls, you can build your frame as big as you want, as long as it has sufficient supports to handle the weight of the tub.