Building a deck is usually always a good idea. Fall season may be one of the best times to build a deck because it's considered the off-season. Contractors may have less jobs during the off-season, thus motivating them to work for less money. They may also be able to complete your job faster since they have less demand. If you live in an area such as San Diego, California, you can probably build your deck any time of the year since the weather is usually very nice.