How to Build an Addition

Building House Addition
Getty / Branko Miokovic

House additions: no other remodeling project creates as much space. Additions are the one house alteration that everyone seems to want.

Because the project is so gigantic, all advice is equally gigantic. If you have ever wanted an abridged "nutshell" version of addition building, here it is:

Come to the Final Decision

  • Know the Scale: When you build a full house addition, you are building a house—it just happens to be a tiny house that is attached to your main house. Everything else is just like building a house: permits, contractors, foundation, electricians, plumbers, framers, roofers, window installers. The list goes on.
  • Know the Price: You might just spend the equivalent of a Harvard education. While some homeowners report paying $50,000 to $75,000 for a full-scale, multi-room house addition, the safest number to rely on will be one that is in the low six figures.
  • Know the Emotional Toil: "The woes of home building" is practically an entire genre of film and literature. Even the best relationships will be taxed during this grueling process.

Secure Funding

Most homeowners cannot pay for full additions in cash. Thus, a loan or line of credit is needed.

Most home renovation projects can be completed with cash. Even if you aren't pulling in a lot of money, you can always break down projects into small chunks so that you can build it - and pay it off - over time.

But the house addition is the one exception: you need a loan. Most homeowners obtain a home equity loan, second mortgage, or line of credit, based on the amount of equity, or value, that their homes have.

Find and Meet With Contractor

Everything hinges on finding a good contractor that you can work with.

Get real, from-the-gut recommendations from neighbors, friends, or relatives. If they cannot recommend a contractor, lose your shyness and knock on the doors of homes that have recently had additions put on.

The importance of the contractor cannot be underestimated. This job is too big for you to contract out on your own unless you have professional experience and ample time.

The first meeting establishes the scale of the project, the contractor's timeframe, general design issues, and cost-saving issues. 

Know that you can ask the contractor how you can control costs during the process. This is your money, after all, and a huge chunk of money, too.

The contractor will take a percentage of the gross costs. So, a $100,000 addition, at an 18% contractor commission, means that the whole cost will cost $118,000. 

Meet With Architect

Except for the most basic design, you will likely need an architect.

Some contractors can design your addition or they may have stock addition plans.

Alternatively, you can hire an architect. There is some value in going with an architect recommended by the contractor. With this arrangement, you have two parties who are accustomed to working with each other.

Even if you eventually choose this arrangement, you should still elicit two or more additional architect bids to determine if the contractor's architect is trying to gouge you.

Permits, Demo, and Prepare Site

Anything other than level bare dirt will need to be demolished, removed, and graded.

A crew will come and drop off a portable toilet, and perhaps put up a sign telling the world who is building your addition.

Your contractor will obtain permits and will be required to post the approved permits in a visible spot on your property.

Obstructions will be removed, even trees (if permitted by your community). Fences will be temporarily taken down to allow heavy equipment to access the site.

Foundation Building: Slab, Basement, or Crawlspace

Just like a real house, the addition will get a full-scale foundation.

Depending on the plans discussed with your contractor, the crew will begin pouring a concrete slab or digging out a crawlspace or basement.

Foundation footers—the peripheral base upon which the addition will rest--are poured or constructed of block concrete.

Framing: Walls Go up

As soon as foundation concrete is cured, exterior walls are built.

One day you come home from work and, suddenly, your addition has two, three, or even four walls up!

The basic, framed walls go up fairly quickly. In some cases, these frames are even constructed off-site.

The walls are still open-air, exposed stud "skeletons." Covering comes later.

At this point, you feel like the project is merely a few days from completion, though you still have a long road ahead of you.

Wall Panels and Roofing

Wall panels and roofing are necessary to protect all work that will come after.

Wall panels, usually of OSB, are installed quickly, with house wrap such as Tyvek on the outside of the panels.

As the house is completely roofed, the project appears to be moving ahead at a fast pace.

Vital Systems: Electrical, Plumbing, HVAC

Crucial services like electrical, plumbing and heating/cooling slow down the project once again.

Project snag or project-as-normal?

It is normal for the project to appear to slow down when electricians, plumbers, and HVAC technicians come in and work their magic. 

These tradesmen, who represent your biggest expense, tend to work fairly rapidly. The snag tends to be with waiting for city inspectors to come in, inspect, and approve.


New-construction windows are installed. Like roof and exterior walls, they further "button up" the structure and keep it weather tight for subsequent work.

Replacement windows go in existing buildings. Because this is an entirely new building, new-construction windows are added.

Insulation and Drywall

With the addition of Insulation and drywall, the addition is beginning to look like a real structure.

Your addition must be insulated. Roll fiberglass insulation is installed between studs in the walls.  Batt fiberglass or blown-in insulation is added between the ceiling joists.

Drywall is a multi-stage process: hanging the sheets, "mudding" the seams with wet drywall compound, letting that compound dry, then sanding the seams.  

Flooring and Paint

Flooring might be installed before paint is applied or paint may come first.

Usually, it is a toss-up as to which is the most effective method (in terms of cleanliness), so this is often dictated by scheduling.

Paint contractors are experienced at painting cleanly after finish flooring has been installed.

Trim Work and Doors

The carpenters come in and put up detailed trim work such as baseboards, window trim, crown molding, and so on. Doors are hung.