Hello @SMGrays
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your timber floor.
Installing engineered timber floating floorboards over an existing timber floor is generally feasible, especially if the subfloor is level and in good condition, as you've described. Since you've already laid the new flooring in one bedroom and observed a small height difference, it's understandable that you're concerned about adding more thickness.
If your existing timber floor is mostly level, sound, and free of squeaks or movement, you can likely install the floating floor directly over it using your 3mm acoustic underlay. This approach minimizes added height and avoids the complexity and cost of additional layers. Before laying the underlay and floorboards, make sure the existing boards are securely fixed by screwing down any loose ones, and that the surface is clean, dry, and relatively flat. Minor imperfections can be absorbed by the underlay, but significant dips or bumps may need sanding or levelling compound.
Adding a plywood or MDF layer is typically recommended when the subfloor is very uneven, unstable, or made of varying materials and thicknesses. This creates a uniform surface, but also adds 6–9mm of extra height, which could further complicate transitions to other rooms or affect door clearances. Given your concerns about height, it would be best to avoid this unless your current floor has problem areas that would cause long-term issues for the floating floor.
Your 3mm acoustic underlay should offer a good balance between sound insulation and minimal added height. Keep in mind that transitions between rooms can be smoothed with reducer profiles or threshold strips, and doors may need trimming to accommodate the new floor height.
In your case, proceeding without the extra plywood or MDF layer seems reasonable, as long as the existing timber is stable and flat. Just be diligent about preparing the surface beforehand, and you should be able to achieve a smooth, durable result without introducing excessive height.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
