1- Know your property.
For the majority of projects that involve a structural change, whether "up" or "out", a full survey by a licensed surveyor is a "must-have". In Seattle, the fee to have a property surveyed averages between $2,500 and $3,000 for a typical urban residential lot and up to $5,000 for larger or more complex lots, depending on factors such as the distance to nearest recorded monuments, the presence of environmentally critical areas (steep slope, known landslides, etc.), and the quantity and complexity of the existing improvements.
For more information about obtaining a survey, including a list of required information and additional items that can save you time and money to have recorded simultaneously, Tips for Getting the Best Survey the First Time.
2- Be a good neighbor.
The golden rule "Do unto others as you would have them do unto you" holds true. If you have been opinionated and difficult in the past about your neighbor's projects, they are more likely to act the same when it is your turn. If you've had troubles in the past, it is a good idea to try to repair relationships in earnest long before you begin the design process.
3- Communicate.
If you plan to change the appearance of the exterior of your home in any significant way (including paint color), it makes sense to let your neighbors know your plans. You are not (necessarily) asking for permission, rather letting them know what kind of disruption to anticipate and how they should handle any issues that arise, such as blocked access, property damage, etc.
4- Know your rights.
Sometimes, questions arise which are difficult to answer with certainty. If your property has easements, encroachments, or other complexities involving property lines, it would be wise to do some research. This may be in the form of a feasibility study, a binding meeting with the relevant parties at your land use department, and/or legal advice from an attorney who specializes in residential real estate law. As an architect, I advise my clients when such additional measures are appropriate. It is less expensive to secure your footing in the beginning than it is to fight your way through a dispute when you are in construction.
5- Have an alternate plan.
One of my former clients planned to add a porte cochere, a historic type of carport, onto the side of his house. His property was located in a historic preservation overlay district, where many of the neighborhood homes had this feature. There was plenty of room on his property, and the structure was permissible under the current zoning code without requiring a variance. However, due to the overlay district, design approval was required from a separate committee, prior to permit approval. When we presented the design for consideration, the neighbors attended the meeting to lobby against the porte cochere - not because they didn't like the idea or design, but because they didn't like my client!
In the end, we did not get approval for the covered carport, but we did accomplish the goal of providing off-street parking by capitalizing on the fact that the committee did not have purview of driveways, curb cuts, or fences. Our "Plan B" was to install tire strips and a parking pad made of permeable pavers. The homeowner now parks where he had originally hoped, and walks up a set of side stairs directly onto his porch, getting out of the rain quickly. When there is no car parked in the driveway, the permeable pavers look like a grass lawn.
Parking beside the porch steps was a viable alternative to a porte cochere.
6- Offer up something.
When all else fails, offer up something. In the past, I've known clients to share or carry the expenses on things such as tree removal or pruning, new fences, landscaping, privacy screens, sewer repair, and burying overhead cables. All of these things cost money, but they are often items which were desired - or would soon have been required.