Almost everyone who asks me this has already decided the site looks dated, and wants permission to spend money on it. That is the wrong test. Plenty of ugly sites make money and plenty of beautiful ones are quietly broken underneath.
The test that actually works
Try to make a small change. Add a page. Change a price. Put up a banner for a promotion that starts on Friday. Time how long it takes and count how many people you had to ask.
If that takes ten minutes and nobody else was involved, your site is fine and you have a design problem. A redesign — new layout, new type, better photography, clearer buttons — will fix it, and it costs a fraction of a rebuild.
If it took three days, or you gave up, or the answer was "we have to ask the guy who built it and he does not reply" — the design is not your problem. You have a structural problem wearing a design problem as a costume.
Signs it is genuinely a rebuild
- Nobody can edit content without a developer.
- It is slow on a phone on mobile data, and the fix is always "add a caching plugin".
- It runs on a platform version that stopped getting security updates.
- Adding a language, or a second currency, means duplicating the whole site.
- The checkout drops people and nobody can tell you where, because nothing is measured.
Two or more of those and a redesign is money spent painting a house with a cracked foundation. The paint will be lovely. The crack will still be there next year, and you will have spent the budget you needed for the real fix.
The middle path nobody offers you
There is a third answer that agencies rarely propose, because it is the smallest invoice: keep the site, fix the three things that are actually costing you money. Usually that is page speed, the contact or checkout flow, and whatever is stopping you from editing your own content.
If someone quotes you a full rebuild before asking what happens when you try to change a price, they are selling, not diagnosing.
Send me the URL and I will tell you which of the three you are looking at. If it is the cheap one, I will say so — a small honest job that goes well is worth more to me than a big one you resent paying for.