The trick is to get them to realise you can't polish a turd, without actually saying it. Hypothetically, if they had a current website, would you be able to leave it lying around in your next post?
There are three main problems here, firstly they may not have the budget to start from scratch. Secondly, they may not have the time or want to start from scratch. Thirdly, they may not have a clue about web development and, for example, may just assume that once you build a website, you just add to it, and that there's no need to shell out for a new site.
1. Find out what sort of budget they are looking to work with, get a range (min/max) - if they don't want to tell you, it will be necessary to explain that you can work with budgets of £1,000 or £20,000 so you need to know where abouts on the scale they sit. If they still don't tell you, well it's a big red flag and you may want to consider whether you really need to work with them...
2. Find out what they want to achieve. What industry are they in, how important is it for them to have a website and what do they really need on it? Do they want to promote their brand, engage with customers/clients and generate interest their services/products or do they just want a website so that they can say they have one..?
3. Explain to them your process for reaching the goals they want to achieve - which hopefully will require more than just a quick polishing of the design - and then explain the difficulties involved in having to do that with their current site.
The more structure you bring to the table, the more professional you'll look and the more likely they will be to take on board your recommendations. Beware though, if its a new client, they will probably be wary of people trying to get them to buy things they don't need. I always sit down for at least an hour with new clients and really get to know them - that way there is trust before you start discussing what services they need.
