If you already have a website you may wonder if it is possible to improve the SEO and marketing of it without having to have a completely new site. If you have to have a new site then this will obviously add additional costs on to the project which you may not be able to afford right now.
Some websites can be worked on and improved without the need to start again whilst others it may not be possibly or even cost effective to try and change, in which case, you will need to have a new site. If for example your website has been hard coded and have no SEO elements then a web agency or individual who is going to do the marketing for you may advise you that they cannot easily work with it. It may actually take them longer to add in all the relevant bits they need to make the improvements than to actually just start again. It may be that a lot of the content and images that you have on the site can be reused, so often it may not be a huge upfront cost to have it rebuilt. Also, some agencies will be willing to split the project cost up over several months making it more affordable.