Creating a bathroom addition means you will need to add new plumbing to both the sink and the toilet. This is a complex job that requires plenty of planning and plumbing skills. You need to run water lines and drain lines for both fixtures. The plumbing alone will take 3 to 4 days or possibly more if you have a slab construction house where it can be difficult to access the drain lines.
Step 1 - Plan
Your bathroom addition should be as close to the main waste line of the house as possible. This will make the plumbing a lot easier as you won’t need to break into as many walls or take up so many floors. For the toilet, you’re going to need to add a waste line that’s at least 3 inches in diameter.
You also need to make sure you have enough space in your bathroom addition to satisfy building code requirements. It will need to be an absolute minimum of 66 inches long by 30 inches wide if the sink and toilet are on opposite sides, and 54 inches by 48 inches wide if they are side by side. Once you’ve established the dimensions of your bathroom addition, sketch in the plumbing you’ll need and where you’ll put it.
Step 2 - Start the Addition
To start the bathroom addition, you need to frame the walls although you shouldn't add any drywall yet. With the framing in place, you can begin to add the plumbing. Remove the flooring to expose the subfloor. You can leave the subfloor in place by only cutting where you need to. If there’s a joist in the place where you have planned on putting the toilet, you’ll need to re-think your plans and move the toilet slightly.
Step 3 - Add Plumbing
Start the plumbing by laying the waste pipe. You will need to go from the bathroom addition down to the main waste pipe for the house. When connecting the sections, use purple primer on each one before gluing them together. Where the toilet will sit, you'll need a toilet flange that will attach to the toilet. To make sure it's at the right height, screw tile backer board to the joists then slide two tiles over them. Finish by resting the flange on the tiles.
Remember that you'll also need to run pipe up through the walls to acts as vents for the piping. Run them all the way to the attic and join them to the other house vent pipes.
Step 4 - Add Copper Pipe
You will need to add a copper pipe to supply water to the sink and the toilet (the toilet will be a branch coming off of the cold line). To do this, you’ll need to add new lines onto the existing house lines. Turn off the mains water to the house, puncture the pipe, and then sweat and solder the new copper pipe for your bathroom addition. Wherever possible, keep the copper lines straight and if you need to bend them, use a proper pipe bender.
Where the pipe is within 1¼ inch of a stud or plate, you’ll need to cover that area with a small protective plate made of steel to avoid possible punctures. Once this plumbing is in place, you will be ready to add the sink and toilet by plumbing them in.