One page website - Effect on SEO

If you have only one web page then it will most likely cover several topics (e.g. each web service offered) and thus not be focused on any one topic. This will make it more difficult to get that page to rank well for one specific topic (you only get one page title, one h1, etc).

If you break it down into multiple pages you will be able to focus each page on a specific topic. Additionally, interlinking your pages will also give you a (very) small boost as you will have the advantage of anchor text and PR transfer.