Rate is subject to the condition of the toilet area.
A proper investigation needed to identify the water leaking issue and using the right method to solve it for long lasting 

I guess many home owners experiences the water leaking was fix by contractors but the water leaking comes back after days, weeks, months....

Basically these are the method type of waterproofing repairs at toilet area depending the structural toilet floor condition.

1. PU injection chemical (no hacking)
2. Epoxy grout between the tiles joint (no hacking)
3. Epoxy flake coating (no hacking)
4. Water repellent chemical between the tiles joint (no hacking)
5. Hacking and redo everything new. New waterproofing, new screeding, new tiles.

To each method type waterproofing, there are advantages and disadvantages. 

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Your question regarding retiling is absolutely necessary to change if hollow sound can be heard on tiles
It must be replace as soon as possible because over time as moisture / water will be able penetrating into tiles and structural concrete. This will create big problem in future.

The present of moisture / water over time in concrete will result concrete deterioration that will encourage mold growth, bacteria growth and steel corrosion.
The concrete deterioration means weaker concrete strength and will eventually lead to structure concrete failure.
Structure concrete failure means massive crack inside the concrete.

That is the reason we always say be careful purchasing old house as massive repair needed to be done. Water leaking problem. Waterproofing problem. Crack everywhere. 

mold growth