Important tip: Never allow any contractor, tiler or bathroom renovator replace only the bottom row of tiles or a section of tiling in a shower recess or when replacing a polymarble or a tiled shower base.
Not only will the shower not comply with waterproofing regulations and building code requirements- insurance companies could refuse future claims if non compliant repairs are carried out on your bathroom.
When only replacing the bottom rows of tiling there will be a break and join in the waterproofing membrane barrier.
This will cause the shower to leak as soon as your house experiences the slightest building movement.
The grout between the old and new tiles will crack, and once the grout has cracked water will leak through the join, penetrating directly through the plaster or sheeting. The join in the waterproofing will allow a path for the water to travel.
A shower is never properly waterproofed unless the entire shower recess has been waterproofed in a continuous application and lipping or curving
up the walls to a minimum of 1800mm height. It is simply impossible to join waterproofing to another section of waterproofing if there are tiles still intact- anybody claiming to be able to do this will be doing a shoddy job.
We regularly come across many handyman and DIY jobs where only the bottom row of wall tiling (instead of the whole shower) has been replaced with a shower base replacement, and instead of getting the job done properly the first time, home-owners are having to pay a lot more to fix the structural damage that the leak has caused from shoddy non compliant work rather than having it done properly in the first instance