One thing my house is missing is a nice, large utility sink. The small sink in my downstairs bathroom is not built for cleaning shoes, tools, and sports equipment. A new sink requires new water and sewer hookups. Lets take a look at what it takes to a sink to a home.
1) Required hookups
Hot and cold water
Hot and cold water supplies are the easiest pipes to route. The water in the pipes is under pressure and can go almost anywhere in home. Be mindful of corners in your lines and the total length of the pipe. The pressure in the line will drop with every foot of pipe and every corner in the line. Vertical runs do not help either.
Drain
What comes out of the tap must go down the drain. Drain pipes can be the most difficult pipes to install, because they depend on gravity. Drain pipes must maintain a minimum pitch all the way to the discharge point.
The water requirements and intended use of the appliance determines the required drain pipe diameter. Toilets require the largest pipes; vanity sinks and showers the smallest. Consult the plumbing code for the requirements for your project.
Vent pipe
One thing people tend to forget is the vent pipe. Every drain connection comes with the requirement to connect to a vent. Besides not being code compliant, a vent-less drain does not drain properly. The air trapped in the line has to escape. Since it can't, it will interfere with water draining through the pipe. To make matters worse, a vent-less drain can create suction in other parts of the system. If that force pulls the water from the p-traps in your sink or toilet, sewer gas can enter your home. That's not a good thing.
The plumbing code calls for a drain to be connected to a vent within 5 horizontal feet. Larger pipe diameters allow for longer distances. Certain pipe layouts can also extend the distance to the vent stack. Creating a code compliant vent is a necessity.
2) Location
Depending on the location it may be easier or harder to install a sink. Most homes have one or more plumbing stacks. You may notice how all water related locations in your home are near each other. For example, the upstairs bathroom is directly above the downstairs bathroom. This is done to allow the different locations to share the same vent and drain stack. This saves money on pipes, and requires only one vent protruding through the roof.
We can learn from this. A location close to an existing water related appliance will be close to the required supplies. It will be easier and cheaper to install a new appliance near such a location. The other side of a walls that host a shower or sink are great places to consider.
Other locations will require all the necessary hookups to be piped in. That's easy enough to do for the supply lines. The larger diameter drain pipe and the vertical vent stack are harder to accommodate.
3) Pressure drop
Just because there are pipes in the wall does not mean it is a good idea to feed a new sink from the lines. Tapping into existing pipes changes the dynamics of the system. Individual pipes do not carry and infinite supply of water. The amount of water each pipe can carry is determined by it's diameter and the water pressure in your house. If too many water consumers are fed of a single line, the pressure at each point of consumption changes.
The sudden change of water temperature in some showers when someone flushes a toilet is an example of this. When the toilet is flushed it consumes water to refill the tank. There is less cold water for the shower to consume, hence the temperature of the water changes (the ratio between hot and cold water in the shower's mixer changed).
In practice this leads to a few considerations. For example, while it is practically possible to feed a new shower off the supply lines of an existing shower, it may not be a good idea. Both showers will compete for the existing water supply. In the morning two people may have their own shower to use, but they may not have enough water. Even if there is enough water any changes to the temperature settings on one shower, will have slight effect on the other.
My utility sink is fed by the lines that service one of the bathrooms. This is a reasonable setup, since these two system are used at different times. In morning when the shower and bathroom sink are in high demand, the utility sink is mostly unused. During the day, when utility sink is used, the shower is not. At worst the sink will interfere with someone washing their hands in the bathroom.
The plumbing code imposes regulations on how many points of consumption can be fed by a single supply line.
4) Doing the work
Doing the work can require a multitude of skills. Supply lines in most homes will be made of copper. Extending the system will require basic soldering skills.
Newer homes may have supply lines made from PEX tubing. These plastic tubes are arguably more DIY friendly than copper.
The drain stack also comes in may different materials. PVC is the easiest to work with and, thankfully, most common in newer homes. PVC fittings are available at home stores.
Older homes may use cast iron or copper stacks. Copper drain pipe is very expensive, but only as hard to install as copper supply lines. Cast iron requires lead and oakum joints and large, special purpose tools.
None of these materials necessarily takes the project out of the reach of a determined homeowner. The older materials just make the project take more time and care.
Summary
In general it is easier to add a sink or shower close to an existing sink in your home. The necessary pipes are already in the wall and only need to be tapped into. The further way from the drain stack the system becomes more complicated to design and install. If you plan your project carefully, adding a sink to your home is a DIY project.
