Why bathroom renovators Always Go Over Budget

If you've heard someone say their bathroom renovation came in on budget, they're either lying or they got extremely lucky. Bathrooms are the single most likely renovation project to blow past the quoted price. The average Australian bathroom reno costs $15,000–$35,000, but cost overruns of 20–40% are so common that builders practically expect them. Here's why it happens.

Waterproofing failures hiding under tiles

The moment your tiler pulls up the old floor tiles, there's a meaningful chance they'll find waterproofing that's failed, degraded, or was never properly done in the first place. In Australia, waterproofing is a regulated trade in most states — it must be done by a licensed waterproofer and inspected before tiling can begin. Remedial waterproofing costs $1,500–$4,000, and if water damage has reached the subfloor or framing, the cost of structural repair can be $3,000–$10,000 or more.

This is particularly common in homes built during the 1970s–1990s, when waterproofing standards were less rigorous. The frustrating part is that there's no way to know the condition of the waterproofing membrane without demolishing the existing bathroom first.

Drainage and plumbing relocation

Moving a toilet by even 500mm can be a surprisingly expensive exercise. Toilet waste pipes need a specific fall gradient (1:60 minimum in most Australian standards), and if the new position doesn't work with the existing drainage, you may need to jackhammer the concrete slab to reposition the plumbing. On a concrete slab, this costs $2,000–$6,000. In a raised timber floor, it's usually cheaper ($800–$2,500), but you may uncover rotted joists from old leaks.

Tile allowances vs. reality

Most bathroom renovation quotes include a tile "allowance" — typically $40–$65 per square metre. That's enough for a basic porcelain tile from a big-box store. But once you walk into a tile showroom and see the options, it's very tempting to upgrade. Feature tiles, large-format tiles, natural stone, or imported European ceramics can cost $80–$200+ per square metre. On a typical 15m² bathroom, the difference between the allowance and what you actually choose can be $1,500–$4,000.

Add to this the cost of more complex laying patterns (herringbone, stacked bond) which take more time and generate more waste, and your tiling bill can easily double from the original estimate.

Fixtures and fittings price shock

Tapware, shower mixers, toilet suites, vanities, mirrors — they all look similar on paper, but the price range is enormous. A functional mixer tap starts at $150, but once you start looking at matte black, brushed brass, or other designer finishes, you're at $350–$800 per piece. Multiply that across a basin mixer, shower mixer, bath spout, and towel rails, and the total difference is significant.

Builders' quotes often include "PC sums" (prime cost sums) for fixtures, which are educated guesses at what you'll spend. These are almost always too low, because they assume you'll pick from the bottom or middle of the range. Most homeowners don't.

How to actually stay on budget

The most effective thing you can do is choose every single fixture, tile, and fitting before the contract is signed — not during the build. Get actual prices, not allowances. Visit showrooms, get exact product codes, and have your builder quote against those specific items.

Add a 20% contingency to your total budget for unknowns behind the walls. If you don't use it, great. If you do — and you probably will — you won't be caught off guard. Finally, lock in a fixed-price contract where possible, and make sure it specifies exactly what happens when variations arise.