How to connect Shower tray drain to Wet room Floor waste?

In most cases you won’t be able to connect a shower tray directly into an existing wet room floor waste without opening the floor.
Wet room drains are designed to take water directly at floor level with the correct falls built into the floor. A shower tray waste needs its own trap and pipework with the correct fall to the drainage system.
Trying to connect into the floor drain from above usually isn’t practical and can lead to issues with flow, smells (trap problems), or leaks.
Proper options would be: • Lift the floor locally and connect the tray waste into the existing pipework below
• Or run a new waste pipe from the tray to a suitable connection point
If you’re trying to avoid breaking the floor, sometimes a raised tray with accessible pipework can work, but it depends on layout.
Realistically, for a proper long-term job, some level of floor access is usually required.

Usually, not neatly and not reliably.

A shower tray waste normally needs its own trap/waste connection directly below the tray, and wet-room drains are built around the floor, forming a gradient for drainage and waterproofing of the wet-room system. In other words, a tray waste is not normally designed to just “plug into” an existing floor drain from above without access to the drainage below.

So in practice, you have 3 realistic options:

1. Best option: open the floor locally and connect properly
That is the standard way. Wet-room and tray installation guidance assumes access is needed to align the waste outlet and drain connection correctly.

2. Use a raised shower tray on a riser/waste kit
If you do not want to break the finished floor, a raised tray can create enough space beneath it for the tray trap and waste run. Bathroom guidance notes that trays sometimes need to be lifted off the floor so the waste can run away properly to the nearest drain point.

3. Use a pumped waste system
If the floor is effectively "unbreachable,” a pumped drain/shower waste pump is a recognised solution specifically for floors where normal gravity drainage cannot be achieved.

What I would not recommend is trying to:

push the tray waste straight into the floor drain opening from above,
rely on a flexible hose hidden in the floor,
or seal one waste fitting onto another without proper access.
That usually leads to poor flow, trap problems, smells, or leaks. This is an inference from how tray wastes and wet-room drains are designed as separate systems requiring proper drainage alignment and waterproofing

Ideally you need to dig out the floor so that the new shower waste connects directly into the original waste pipe

To connect your shower tray waste to an existing wet room drain without breaking the floor, you'll need to lift some flooring to access the existing gully/pipe, use flexible connectors or solvent/compression fittings to join the new tray waste to the existing pipework, ensuring the trap is correctly positioned and sealed, then refill and screed around the tray to create a flush, watertight finish, potentially using tile backer boards for height.