Bathroom Remodel: The Plumbing Decisions to Make Before Demolition
Everything on this list is straightforward while the walls are open and painful once the tile is on. Decide it before the first swing.
July 6, 2026 · 7 min read
A bathroom remodel is one of the few moments when your plumbing is genuinely accessible. Walls are open, floors are up, and work that would otherwise mean demolition is simply part of the job. That window closes the moment the tile goes on.
Almost everything below is inexpensive while the walls are open and expensive afterward. Decide it beforehand.
Decide the layout before anything else
Moving a toilet is the most consequential decision in any bathroom remodel, because it means relocating a drain and a vent — and in a slab home that means opening concrete. Moving a sink or a shower is easier but still involves drain work.
If the budget is tight, keeping fixtures in their existing locations saves a substantial amount. If you are moving them, decide now and get it priced properly, because a layout change discovered mid-project is where budgets come apart.
Look at what is already in the walls
Once the walls are open, you can see the supply and drain lines serving that bathroom. This is a free inspection you will not get again.
- What material is the supply? Galvanized steel or polybutylene visible here means the rest of the house probably has it too, and this is the cheapest moment to replace at least this section.
- What condition are the drains in? Cast iron with heavy scale, or an unvented fixture that was never done properly.
- Is the venting correct? Bathrooms remodelled by previous owners frequently have improvised venting that causes gurgling and slow drains.
- Are there signs of past leaks? Staining on framing or subfloor is worth understanding before it gets covered up again.
Rough-in decisions worth making now
Shower valve type and depth
Set the valve body at the correct depth for the finished wall thickness — tile, backer board, everything. Getting this wrong means trim that will not sit flush, and correcting it means opening the wall again. Also confirm the valve is pressure-balancing or thermostatic; it is a scald-safety feature and generally required.
Shutoffs for everything
Every fixture should have an accessible working shutoff. Put in quarter-turn ball valves rather than old-style multi-turn stops — they seal positively and they still work in fifteen years.
Blocking behind the walls
Add solid blocking for grab bars now, even if you are not fitting them. It costs almost nothing while the walls are open and it means a grab bar can be installed properly later without opening tile. On any bathroom intended to serve long-term, this is the easiest good decision on the list.
A bidet seat supply
If there is any chance you will want one, running a supply now is trivial. Adding it later means a visible tee off the toilet supply.
Drain size and slope for a curbless shower
Curbless and low-threshold showers need the drain and slope planned into the structure. In a slab home that is a significant decision with real cost implications, and it cannot be improvised afterward.
Fixture selection notes for this water
Hard water shortens the life of every seal and cartridge in the house, so how you choose fixtures matters more here than in a soft-water region.
- Favour brands with replacement cartridges you can still buy in ten years. In this water you will be changing them, and an obsolete valve is how a dripping shower becomes a tile-out.
- Write down the make and model of the shower valve somewhere you will find it — inside the vanity door works. It saves an hour of detective work next time.
- Be cautious with very inexpensive fixtures. Plastic internals do not last in this water.
- Very high-flow showerheads and body sprays need the supply capacity to feed them. Worth checking your pressure and pipe sizing before specifying a system that the house cannot supply.
The whole-house items to fold in
Two things worth doing while a plumber is already on site.
Check your static water pressure. If it is running high, that is quietly shortening the life of every fixture you are about to install, and correcting it with a pressure reducing valve protects the whole house. It is a small addition to a project that is already underway.
And if the walls being open have revealed polybutylene or galvanized supply, get a repipe quote. Doing the whole house while one bathroom is already torn up is meaningfully cheaper than doing it as a separate project later.
Permits
A remodel that relocates fixtures, adds them, or alters supply or drain lines generally needs a permit in the cities we work across. Like-for-like fixture swaps typically do not. The inspection covers rough-in before it is closed up, which is exactly the work you cannot evaluate yourself.
If you are planning a bathroom and want the plumbing scoped properly before demolition starts, call (469) 338-5533. That conversation is far cheaper than the ones that happen mid-project.
Frequently asked
How much does moving a toilet cost in a slab home?
More than people expect, because it means relocating the drain and vent through concrete. The variables are how far it moves, what is above the slab, and how the existing drain is routed. It is a genuine cost rather than a small adjustment, which is why the layout decision belongs at the start of planning rather than partway through.
Can I supply my own fixtures?
Usually yes, and plenty of homeowners do. Two practical notes: confirm the rough-in dimensions match what is being installed, particularly for toilets and vanities, and check that the shower valve you have chosen is one your plumber can get cartridges for. Also be aware that warranty on a homeowner-supplied fixture generally sits with you rather than the contractor.
Do I need a permit for a bathroom remodel?
It depends on scope and on your city. Swapping fixtures in the same locations typically does not require one. Moving fixtures, adding a bathroom, or altering supply and drain lines generally does. The inspection happens at rough-in, before anything is closed up, which is precisely when errors are cheap to correct.
What is the most common mistake in bathroom remodels?
Waterproofing. Tile and grout are not a water barrier — the membrane behind them is — and a shower pan built without proper waterproofing leaks into the structure invisibly for years. Ask what system is being used and insist on a flood test of the pan before tile goes on. It is a day of waiting and it is the difference between a shower that lasts twenty years and one that rots a subfloor.