Modular Home Additions and General Contracting

Modular Home Additions Need a General Contractor

When building a modular home, it is recommended that you hire a general contractor (GC) with modular-construction experience. In some respects, this advice is even more important when building modular home additions. There are usually a number of surprises when building an addition, regardless of the type of construction. Most of them derive from the fact that you are connecting a new structure to an existing structure that was not specifically designed to accept it. Surprises are typically more frequent and complex with an older existing home. Construction surprises almost always cost money and time, and they can cause personal stress, especially if you remain in your home throughout the project. The best way to manage the challenges of building an addition is to have a professional GC directing the activities.

The Scope of Work for Modular Home Additions

If you are able to build a modular home addition, you will need to work with each of your dealer candidates to determine a floor plan, specifications, and price. The steps will be essentially the same as for building a single-family modular home, except that you will probably need at least one of the dealers to help you create a custom plan. Although most manufacturers build additions, few offer standard plans that were created specifically for this purpose. Many standard modular house plans, however, can do double duty as modular home additions. For example, small ranches can serve as in-law apartments, and the second story of an appropriately sized two-story can work as a second-story addition.
I recommend that you provide prospective dealers with photographs and approximate measurements of the inside and outside of your home. This will help them create a design that meets your needs and fits your existing home. When you sense that a particular dealer can help you, invite him to see your home and take his own measurements.
Take similar steps with your GC candidates. Once you have confidence in a candidate, invite him to visit your home to make sure he can do what you and your dealer are proposing; he should also take his own measurements. He can then finalize what he needs to do to build the addition and present you with a price for his services. Add his price to your dealer’s price and decide if the project can meet your budget.
The GC tasks for modular home additions will be similar to those in building a new modular home. These tasks include completing the site work, foundation, plumbing, electrical, heating, and interior and exterior carpentry. The GC will need to build any site-built structures you need, such as a deck. He will also be responsible for completing some construction tasks that are unique to building an addition, which will be discussed in the following sections.

Septic System and Sewer Hookup

If your existing home has a septic system, you must obtain approval from the local board of health to use your current system with your modular home addition. The determining factors usually are whether your new combined home will have more bedrooms than your existing home and if the septic system was designed to accommodate them. Without the approval, you will not receive a building permit to go forward. The board might give its approval only if you first enlarge or replace the existing system. Even if the system is adequate as is, connecting to it can create some additional expenses. For example, if your addition has a new bathroom and its waste pipe is below the line connecting to the septic or sewer system, you will need a pump.


You should expect that part of your lawn and landscaping will be disturbed by the excavation work. You can preserve some of your shrubbery by relocating it before the construction begins. If you have a paved driveway, it might suffer some damage as well.

Utility Wires and Antenna

If any utility wires are in the way, the GC should arrange for their temporary relocation before work begins. Since utility companies often require a few weeks’ notice, this must be scheduled in advance. In addition, other items attached to your home that may affect the construction of the modular home addition, such as a TV antenna, will be need to be taken down before work is begun and then reinstalled after the addition is complete.

When building modular home additions the electrician can use the existing electrical meter or add a second meter when connecting the addition to the electrical service.
When building modular home additions the electrician can use the existing electrical meter or add a second meter when connecting the addition to the electrical service.

Electrical Work

The electrical service to your home might need to be upgraded, especially if you have an older home with a 60- or 100-amp service. The location of the electrical panel box will influence the amount of work the GC needs to do to connect to the modular home addition. If it is on the opposite side of the home, it will cost more to make the connection. To be safe, the GC should instruct the modular manufacturer to make each electrical run long enough to reach the panel box. Another option is to use a junction box or subpanel. If you are building the modular home addition as a separate apartment, you may want to install a separate electrical service, so that the occupants will receive their own electrical bills. The electrician can do this by installing a dual-meter socket.

When building modular home additions the general contractor should connect the smoke detectors in the addition to the smoke detectors in the existing home.
When building modular home additions the general contractor should connect the smoke detectors in the addition to the smoke detectors in the existing home.

Smoke Detectors

The building inspector or fire marshal might require you to upgrade your exisiting home’s smoke detectors. You should do this even if it is not required. Have the GC outfit your existing home with hard-wired detectors, if it does not already have them. Instruct your modular dealer to coil an extra wire in the basement that connects to the smoke detectors in the addition. The electrician can then pull the wire into the existing home to connect the smoke detectors in the existing home to those in the addition. When the two systems are interconnected, a fire in any part of the home will trigger the alarm in all parts, which is exactly what you would want.

Heating and Air Conditioning

If the GC intends to tie the modular home addition into your current HVAC system (heating, ventilation, and air conditioning), your boiler or furnace might need to be upgraded to take on the additional demand. You might be able to do this affordably if the heating system is slightly oversized and retains some untapped heating capacity; it is not unusual for contractors to provide more capacity than is actually needed. Otherwise, you will have to consider an entirely new heating unit.
This same point holds for central air conditioning. You will only be able to tie into the existing compressor if it has sufficient capacity.
You can avoid the expense of replacing the heating system by using electric-baseboard heat in the addition. Putting electric heat in a second-story addition for a home with hot-water or warm-air heat on the first story might even save you money. Since the second story is likely to be made up of bedrooms, you can take advantage of the fact that the temperature in each room can be controlled separately with electric heat. However, if you have central air conditioning on the first floor, you will likely want to extend the ducts into the addition.
Electric heat can also work well with a small in-law apartment; its small size will keep the heating cost low. If you have a separate electric meter for the apartment, you and the occupants will be able to identify the exact usage.
Another option is to install a separate boiler, furnace, or compressor for the addition. You may want to consider this alternative when your current heating and cooling systems are on the opposite side of the home from the addition and the GC believes the distance will cause too much heat and cooling loss between the unit and addition.

Water Heater

You will have similar issues with supplying hot water to the modular home addition as you will have with the heat and air conditioning. Unless you have excess capacity in your current system or want to upgrade, you will need to add a separate water heater for the addition.
Next week I’ll discuss attached modular home additions.
For more information about building modular home additions, see Building a Modular Addition in my book The Modular Home.