If "Seattle Homes" is already trademarked am I allowed to trademark "Seattle Home" or does the trademark they have include plurals and singulars?

You say that "Seattle Homes" is "already trademarked." What you mean is that someone else is using "Seattle Homes" as a trademark to offer certain goods or services. The standard for trademark infringement is "likelihood of confusion," meaning would a consumer of the services you and the other person are offering confuse the source of those services based on the similarity of the trademark. Almost without a doubt, you will not be able to use a trademark that is identical to someone else's trademark except that you made it singular or plural. However, that really isn't the question. Figuring out if a term like "Seattle Homes" can be used as a trademark is not as simple as seeing if someone else is using the same term you want to use. You have to figure out if the term can be used as a trademark. You have to see if the person already using the term is using it on the same goods/services you want to offer. You have to figure out if the trademark is "inherently distinctive" or has "acquired distinctiveness." You have to see if the person who used it before you has registered the trademark with the state or the US Patent & Trademark Office. Even then, there is nothing to stop the prior user from trying to stop you from using the same or a similar trademark. In the end, the term "Seattle Homes" when used to offer any kind of goods or services relating to homes or housing is very descriptive and is a weak trademark, so even if you use it, you will have trouble stopping people after you from using very similar trademarks. Picking a strong trademark is a better option.