Bathroom Renovation: Planning, Sequencing and Costs

Bathroom renovations go wrong more often than almost any other home improvement project. Not because they're technically complicated, but because the interdependencies between trades are tight, the preparation is underestimated, and the selection of a single installer over a properly coordinated team of trades creates problems that take years to manifest as leaks, cracked tiles, and failed grout.

A bathroom done well lasts fifteen to twenty years. A bathroom done poorly will start showing problems within two to three years. The difference isn't mainly about the quality of the fixtures: it's about the substrate preparation, the waterproofing, and the quality of the installation behind the tiles and under the floor.

Layout Decisions

The most significant constraint in a bathroom renovation is the position of the existing soil stack. Moving a toilet requires extending the soil pipe, which has gradient requirements (typically 1:40 to 1:80 fall). In a first-floor bathroom, the fall needs to be accommodated within the floor depth, which can be tight. Moving a toilet across the room is straightforward in terms of planning but can be expensive in terms of the plumbing work required underneath the floor.

Baths, showers, and basins are connected to the soil stack via branch pipes and can be moved more easily than the WC. The constraint is access to the run below: if you're on a concrete floor, any new drainage route requires cutting the floor, which adds cost and drying time.

Before deciding on a layout, draw it to scale. Most bathrooms are 4-6m2, and a bath (1700x700mm), shower enclosure (900x900mm minimum), WC, and basin all need to fit with adequate space to use them. A 750mm clear space in front of a WC and 600mm for a shower door to open fully are the practical minimums. Don't design your dream bathroom based on what you see in showrooms: the rooms in showrooms are much larger than yours.

If moving fixtures significantly, think about whether you want the renovation to also change the plumbing to a pressurised (unvented) cylinder if you're currently on gravity-fed hot water. The difference in shower performance between a gravity-fed system and a pressurised one is very significant, and if the cylinder is already at end of life or you're renovating anyway, this is the logical time to upgrade.

Waterproofing and Substrate Preparation

This is where bathroom renovations are most commonly done wrong, and where the consequences are worst.

Any surface that will be tiled and is adjacent to a wet zone (bath, shower, basin) needs to be on a stable, dry, appropriately waterproofed substrate. The sequence matters.

Floor substrate. Existing timber floors flex. Tiles on a flexible substrate crack and allow water ingress at the grout lines. Before tiling a timber floor, either board with 18mm plywood (screwed at 150mm centres throughout) or tile onto a decoupling membrane (such as Schluter Ditra) that accommodates movement. Alternatively, lift the floor and install a poured screed, which gives a rigid, flat surface. Cement particle board (Hardiebacker) on a timber floor is another option and is widely used. Whatever you choose, the substrate must be rigid and flat: 3mm maximum deviation over 1.8 metres.

Wall substrate. Plasterboard in wet areas is problematic if water reaches it: it deteriorates and loses structural integrity. Use cement board (Hardiebacker or equivalent) in the shower enclosure and above the bath. Tanking: apply a liquid waterproofing membrane (such as Wedi Subliner or BAL Waterproof Plus) to all surfaces in the shower enclosure before tiling. The membrane should extend at least 200mm beyond the edge of the wet zone. Sealing junctions between wall and floor tiles with waterproof silicone (not grout) is not optional: it's the most vulnerable point for water ingress.

Shower trays and enclosures. Stone resin trays are more expensive than acrylic but significantly more rigid and durable. For a tiled wet room or tiled shower, a pre-formed shower tray former (such as Schluter Kerdi-Shower) provides a waterproof sloped base for tiling. Wet rooms without a former on a timber floor require the floor to be strengthened, tanked fully, and properly graded to the drain.

The Trades and the Sequence

A bathroom renovation involves multiple trades. Understanding the sequence prevents the most common delays and mistakes.

Sequence:

1. Strip out existing bathroom
2. Any structural work (new WC position, drainage runs) with floor boards up where necessary
3. First fix plumbing: new pipe runs, waste runs, re-routing of hot and cold supplies to new fixture positions
4. First fix electrical: new circuits for extractor fan, shaver socket, heated towel rail, underfloor heating (if specified), lighting positions
5. Boarding: cement board in wet areas, plasterboard elsewhere, plywood or decoupling membrane on floor
6. Tanking: liquid waterproofing membrane in wet zones
7. Floor and wall tiling
8. Second fix plumbing: installing bath, WC, basin, shower valve, taps
9. Second fix electrical: fitting extractor, heated towel rail, lights, shaver socket
10. Silicone sealing of all junctions
11. Accessories, mirror, towel rails, shower screens

A single bathroom fitter who claims they can do everything tends to be a plumber who also tiles, or a tiler who also does some basic plumbing. This can work for a straightforward refresh. For a full bathroom renovation, you want a plumber who has done work in bathrooms many times, an electrician for the notifiable electrical work (a bathroom is a special location under Part P), and a tiler who tiles bathrooms specifically. Some of these trades will be the same person if they're genuinely multi-skilled; the point is to check their actual experience in each area, not to assume competence across everything because they offer to do it.

Common Mistakes

Not waterproofing behind the bath panel. The gap between the bath and the floor is where water sits. If the wall behind is plasterboard without a membrane, it deteriorates over years. Tile and tank behind a bath even though no one sees it.

Using grout instead of silicone at movement joints. Any junction between floor and wall tiles, between tiles and a bath or tray, or at internal corners is a movement joint. These must be filled with silicone, not grout. Grout is rigid and cracks at these points within months. Silicone accommodates movement. Choose a silicone that matches the grout colour.

Inadequate extract ventilation. Building regulations require mechanical ventilation in bathrooms: 15 litres/second minimum continuous, 30 litres/second on boost (or 15 l/s with a 15-minute overrun). Most basic extractor fans are under-specified. For a windowless bathroom, a more powerful fan with a humidity sensor is worth the additional cost.

Buying fixtures to a tight timescale. Many sanitaryware ranges and popular tile formats have long lead times from manufacturers. Budget 6-8 weeks from order to delivery for anything that isn't stock. Order before you have a start date confirmed, not after.

Costs

These costs include labour and materials for plumbing, electrical, tiling, and basic fixtures at mid-range specification. Luxury sanitaryware (brands like Duravit, Villeroy and Boch, Hansgrohe) adds significantly. High-end or bespoke tiling (large format stone tiles, mosaic, handmade) adds considerably more. Underfloor heating adds £1,000-£3,000 for electric, rather more for wet UFH.