Here are some factors which influence deck design:
Function - how many people will occupy the deck at one time, how much furniture will be on it. The amount and placement of the furniture will influence the shape and size. Buy the furniture and set it out in a flat area to ensure the new deck will accommodate those people and that furniture.
View – Locate the deck to take advantage of the view.
Sun/shade –Decks need to be designed according to your preference for sun or shade. Year-round residents will want a sunny exposure for a deck in colder months . Very sunny locations may need two decks to take advantage of morning and afternoon shade.
Dining - there should be easy access to the kitchen. It's possible to order sliding screens for windows to facilitate passing food and dishes to and from the kitchen. A screened room allows enjoying the outdoors without the bugs.
Looks - a beautiful deck adds to the appeal of the house and apparent size. A plain house becomes more salable with a great deck.
Lot slope - very sloped lots may have little usable outdoor living and playing area, thus a larger deck may be a good investment.
Privacy - having the neighbors or street traffic looking directly onto the deck or vice-versa detracts from the enjoyment of the deck. The railing can be made solid and/or higher to increase privacy. Consider duplicating or reflecting the style of the house siding or fencing in the solid railing style. A solid rail or privacy screen under the deck makes it seem more solid and increases the apparent size of the house.
Shape and proportions - A simple rectangular deck is the easiest to build and the least interesting. Sometimes the available space or budget only leaves room for a rectangle, but cutting off a small corner or two on a 45 degree angle adds a lot of visual interest. Long skinny decks have less usable space and a higher cost per square foot than a deck closer to square. Curves add a lot of interest and cost.
Placement relative to other outdoor recreation areas - If the customer has other outdoor functions, such as a pool, horse shoe pit, riding arena, the deck can be placed to interact with or view that area.
Multiple levels - if the lot slopes, you can reflect that slope in a multiple decks. Decks with only one step difference in height are a hazard, as guests new to the deck or after a cocktail may not notice the step and step off backward or trip. If you do build more than one level, turn the deck boards a different direction for each level so the step is more noticeable.
Steps - Steps are one of the most interesting and intricate features of the deck, thus they add a lot visually. Their drawback is they take up a lot of usable deck space as you would not put furniture near the top or bottom of the stairs.
Natural features - The shape and size of the deck will make sense and be more pleasing if its design takes into account the existing features of the house and lot. Building up to a rock formation or tree brings that feature into the overall impact of the deck and can utilize the shade from the tree. Building the deck into an inside corner of the house increases shelter from winds and privacy.
Trees - Building around trees is time-consuming and often doesn't give much more usable deck space. If the tree dies, it can be more expensive to take it down when it's within the deck plus the deck then has to be repaired. Some trees, like white fir, shed a lot of sap and make the area under them unusable. Cedar trees give good year-round shade, but if your deck boards are close together, the high amount of debris from this tree collects in the cracks and if not cleaned out, can promote rot.
Expandability - If you plan on needing or wanting a larger deck in the future, plan the whole deck now and build the current one to ease the impact and expense of adding on. How the deck boards run influences how noticeable the extension will be. Adding on at a different level minimizes the difference between the look of the old versus the new.
Rail design options - Pickets, lattice, horizontal boards, solid siding, (t-111 plywood, shingles, fence or siding boards), glass or plexiglass, pipe, cables, lathe-turned round or natural poles.
Curves - curves greatly increase the impact and labor time for decks. Parts of the deck and rail can be curved and/or steps can be built with curves. This is definitely an advanced carpentry technique not to be undertaken by the inexperienced or rushed.
Materials - We use primarily 2x6 construction common (with knots) or "B" grade (hardly any knots) redwood, installed with corrosion-resistant screws designed especially for decks. More rarely we will install part plastic, part wood decking, (composite). If the customer is sure he will paint the deck boards or has a very tight budget, use douglas fir 2x6. The maintenance on painted decks can be considerable as the paint peels and often has to be sanded to get the next coat properly installed. An even more rare choice for decking is tropical wood, of which there are several choices. Typically, the more expensive the wood chosen, the more time and care taken to build the deck. Edges are routed or sanded, wood is selected to show the better side and edge.
Rot resistance - if the deck is to be built within one foot of the ground or where there is little air flow or high moisture content in the ground, use all pressure-treated wood for the structure from the ground up to the decking. Any place where wood touches concrete or dirt, use pressure-treated wood. If the appearance of pressure-treated wood is unacceptable, wood preservative can be applied to nicer-appearing wood, though this does not have the rot resistance of pressure-treated.
Benches - Built-in benches are a nice design feature, plus they add usability to the deck, since they save the need to bring out and store away furniture. They take up less space than movable furniture. Place them on the side without the view. Straight runs are much faster to build than ones with curves or joints.
Most of the above are practical considerations. For a truly beautiful deck, you may need a designer with an fine artistic sense, which is different than fine craftsmanship. Both are needed for a highly functional work of art.