A website needs replacing when it cannot do the job, not when you are bored of the colour. The usual signals are practical: it is slow on a phone, it cannot be updated safely, it does not match the current offer, or it is invisible for terms you should own.
Look at the last six months of enquiries. If people still find you through word of mouth and then struggle to instruct you online, the site is costing you work even if it looks acceptable in the office.
Sometimes the answer is not a rebuild. Navigation, page speed, metadata and a clearer services structure can recover a site that is structurally sound. Rebuild when the CMS, the templates or the information architecture cannot support the next two years.
If you are comparing a repair with a replacement, get both scoped. The cheaper path is the one that gets you a maintainable site, not the one with the lower first invoice.
If you are briefing a project in the South West, you may also want web design, website development or a conversation about the brief.