What actually collides
A replacement bidet seat usually has a deep rear housing for valves, electronics, heating, or the mounting plate. An under-seat attachment has a control arm and body behind or beside the mounting brackets. When the toilet deck rises immediately behind the bolt holes, that hardware may not sit flat or slide far enough back.
BidetKing’s public attachment guide says a relatively flat one-piece bowl meeting the tank near a 90-degree angle can work, while a curved tank-to-bowl transition is unlikely to fit its attachments. Bio Bidet likewise warns that special shapes and French curves can be incompatible. Those are merchant-specific statements, not proof that every curved toilet fails.
Do not rely on the toilet name alone
Mold revisions and compact designs create edge cases. One public electronic-seat guide describes some versions of the Kohler San Raphael as compatible only part of the time and says molding differences can prevent a fit. That is exactly why BidetFit does not maintain a universal “yes” based only on a family name.
Evidence strong enough to buy
- Exact toilet model number, including suffix or revision when available.
- Rear-clearance measurement from bolt centerline to the first rising porcelain.
- Side and top photos showing the mounting deck.
- The exact bidet dimensional drawing, not only the marketing dimensions.
- Written confirmation from the manufacturer or merchant that references both exact models.
- A return policy reviewed before installation or water connection.
Alternatives when a seat will not fit
A handheld sprayer may avoid the seat-housing collision, though plumbing access, local code, hose mounting, and shutoff habits still matter. An integrated bidet toilet is another category, but it replaces the entire fixture and requires a different budget and installation decision.