The most basic answer to this question about designing your own bathroom, is that it is your home, it is a personal space, and you are best qualified to determine your specific needs in a bathroom design.
You may be planning to contract the work out, but even so, you should formulate a rough design, a list of what you want in your bathroom, and you should do some shopping to find out what fixtures and materials are likely to cost.
By doing this, you can better convey to a contractor just what it is you want, and you are more in control of your remodeling budget because you have a grasp of what materials should cost and when you are being charged too much.
Your plan will vary depending on whether it is a main bath used by both family and guests, or whether it is a private master bath.
Consider the space you have, and if it is adequate.
If your current space is not large enough, is there other space you can incorporate into your new bath? Look at how your house is laid out and see if there isn't space in a room that would be better used as an addition to your bath.
Sometimes a closet will be located in just the right place to make a good addition.
Rearranging an existing bathroom can add to the cost, when you consider that changes in the plumbing may be necessary, however in some older homes where the original bath was poorly planned in the initial design, making a change can sometimes pay off handsomely in the addition functionality it adds to a small room.
Sometimes just rearranging fixtures and cabinetry in the existing space can add a more roomy feel and make the room much more functional.
Decide if any of the existing fixtures are still in good enough condition to use, and make a list of what you will need.
Bathtub-shower combinations are less popular these days, but if you want both and space is limited, it can be a solution.
This is just one of the decisions you will need to make.
What upgrades do you want? There are many new amenities available for bathrooms, and you should do a lot of browsing before you make any final decisions.
Before you invest in a whirlpool bath or a two person tub for your new bathroom, do some serious thinking about whether you will really use it enough to make the investment worthwhile.
You may be better served with a great new shower head and a temperature control device that prevents fluctuation in water volume and temperature.
Whirlpool baths use a lot of water as do two person tubs which may require special size piping and a larger hot water tank.
Also before installing a whirlpool research the installation thoroughly and ask questions.
You will need access for maintenance and you also want to consider the noise factor.
Bathrooms, for a small room, need quite a bit of planning so you get the best of what you want.
And who knows what you want better than you? Planning your own bathroom design is essential if you are doing the remodeling yourself on a do-it-yourself basis, and can be a great help and money saver if you decide to contract the job out.