The short answer
Usually one site. Build the cosmetic work as its own section, with its own landing page, its own treatment pages, its own gallery and its own booking route, so cosmetic enquiries are separated from check-ups in your diary and in your reporting.
A second website is the right answer in a narrow set of cases, listed further down. For most practices it doubles the maintenance and halves the search authority for no gain.
Why one site usually wins
Everything a new cosmetic section needs, an established practice site already has. Years of search history, reviews, a local reputation, existing patients, and a domain search engines already trust. A new domain starts at zero and takes a long time to reach the same place.
There is also a trust argument that people underestimate. A cosmetic page attached to a real practice with named clinicians, a registration list and a decade of routine care behind it is more credible to a nervous veneer patient than a standalone brand with a gallery and nothing else.
And your own patients are the first market for the new work. On one site they meet it naturally.
What the cosmetic section actually needs
Its own landing page. Not a paragraph on the treatments page. A page that speaks to someone choosing to spend thousands on their appearance, which is a different reader from someone booking a hygiene visit.
A page per treatment. Veneers, composite bonding, whitening, implants, smile makeovers. Each with its own from-price, finance line, gallery and questions.
Its own gallery. Cases held per treatment, with consent records attached, not a single mixed folder.
Clinician credentials front and centre. Who does this work, their training, and how long they have been doing it. Cosmetic patients read this more carefully than any other page.
Its own booking route. A cosmetic consultation appointment type in your practice software, exposed online, with an intake form that asks what they want and what has bothered them, and enough time allowed for the conversation. Not a fifteen minute check-up slot.
Separate the diary and the tracking
This is the part practices skip and later regret. If a cosmetic consultation books as a standard appointment, you cannot tell how many cosmetic enquiries the site produced, which treatment page produced them, or what they converted at.
Create the consultation types separately in the practice software. Tag the booking route so each treatment page's Book button records which page it came from. Then a monthly look tells you whether the implant page or the bonding page is doing the work, and where to spend next.
Where the navigation goes
Give the cosmetic section a single top-level entry in the main menu, with the treatments beneath it. Keep the routine care journey untouched. A returning patient booking a hygiene appointment should not have to walk through smile makeover copy to do it.
Design can shift within the section, a calmer and more considered feel, without becoming a different website. Same logo, same header, same footer, same contact details.
When a second site is justified
A separate legal entity or premises. Different company, different address, different registration. Then it is a different practice and it needs its own site.
A distinct brand you intend to build. A named implant or smile clinic operating with its own identity, its own clinicians and its own marketing budget, deliberately kept apart from the general practice.
A different regulatory or market footprint. Different country, different rules, different language.
If you cannot point at one of those, one site with a strong section is the cheaper and better answer.
The mistake to avoid either way
Do not build a thin microsite on a new domain that duplicates content already on the practice site. Two pages competing for the same search on two domains you own is a self-inflicted wound. If you do split, the sites need genuinely different content and genuinely different purposes.
Where this sits
A cosmetic section with its own landing page, treatment pages, consented gallery and separate consultation route is how we normally add cosmetic dentistry and implant work to an existing site, rather than starting a second one, and it fits within the wider dental practice websites we build. Package details are on pricing. Keep the authority you have and give the new work its own room inside it.