The best starting point for an addition is a wish list. Start the list by asking what problems you want the project to solve (more space) and what goals you want it to achieve (better indoor-outdoor access for entertaining). Once you've made the list, rank the improvements in order of priority. Knowing the end result will help your builder or contractor plan accordingly for items such as wiring, plumbing, and location.
Look into the legal restrictions of what can be built on the property. Most cities have setback restrictions that govern how close a structure can be built to property lines, height restrictions, building area ratios, design covenants, and historic-district preservation ordinances.
When designing your new space, be open to new possibilities. For example, in a new bathroom be open to moving your vanity to a new location -- not just its previous location. Being open to change allows you to use your new space to the best of its ability.
Though additions are about getting extra room, they don't need to be large. As you plan, consider stealing space from adjacent closets or hallways to keep your addition at a modest size. In the kitchen, a small breakfast nook only requires a few feet of space, yet it can transform the entire kitchen.
An angled arrangement offers opportunities to add interesting exterior elements. Position a breakfast room at an angle to help break up a rectangular facade.
The problem with building on a sloped lot is that any structure risks appearing like a room on stilts. Take a cue from the vertical orientation and stack multiple spaces for an addition that blends into the hillside and provides plenty of space for the homeowners.
A perfect siding match can help make a new addition look like it's been there all along. But such cloning of original surfaces isn't always possible or even desirable. Choosing materials of the same vintage and tonal range but with slightly different textures, for example, creates a pleasing harmony that respects the old while setting off the new.
Mimicking the style and pitch, or angle, of a building's original roofline helps an addition fit in with the original structure, whether you're building up or adding an entryway.
Repetition of architectural features is key to a well-designed addition. Otherwise unnoticeable elements can clash, if they're of two completely different materials or design styles. Carefully consider all building elements on the original structure, including windows, trim, doors, gutters, lighting, and hardware. If the house features double-hung windows, for instance, don't install swing-out casements in the addition.
An otherwise well-designed addition can look like it was hastily tacked on if the plantings, walkways, beds, and borders that surround it don't follow the style of those around the original house.
You don't want your new master suite to look like the addition that ate the house. Nor do you want to add a mudroom so small that it looks more like a toolshed than an entrance. Keep things in proportion.
Concern yourself not only with how the addition will look from the outside, but what the outside will look like as viewed from the addition. Design, place, and orient yours to take advantage of striking views -- and so that it will look good when viewed through the windows of the original house.