What to avoid when renovating a bathroom?

Avoid cutting corners on the things that disappear behind the tiles: waterproofing, the substrate, ventilation and the electrics. Every expensive bathroom failure we get called out to in South East London starts in one of those four places, and fixing them after the event means ripping out finishes someone has just paid for.

Tank the wet areas properly. Tile adhesive and grout are not waterproof, and a shower used daily will find any gap within a couple of years. The same goes for substrates. Victorian and Edwardian houses in Lewisham and Crystal Palace often have blown plaster or layers of old paint under the surface, and tiling straight over that means the weight eventually pulls everything off the wall. We strip back and board wet zones with cement backer board, every time.

Ventilation gets skipped because a fan feels optional. It is not. Fit a decent extractor ducted to the outside, not into the loft, or you are growing mould from month one. And never let anyone unqualified near the gas or the electrics. Bathrooms have defined electrical zones with strict rules about what can go where, so use a registered electrician who certifies the work, and a Gas Safe engineer for anything touching the boiler or gas pipework. Layout and fixture choices are a separate question. Get the fabric and services right first and the rest is decoration.