Home addition: what to know

What the job involves, what changes the price, and what to ask a pro.

What home addition is

A home addition adds new living space to a house by building a new foundation, framing, roof, insulation, utilities and finishes, then connecting it to the existing home. People need one when the house is too small for the family and moving is not the right option.

Signs you need it

  • There are not enough bedrooms or bathrooms for the people who live in the house now
  • A room is too small for the way it is used, and you want to extend the footprint instead of changing the layout
  • The existing kitchen or living space is cramped, and you want a larger room that opens to the yard
  • The house has a layout that cannot be reworked to add the room without building outward
  • You want a first-floor bedroom, an accessible suite or a home office that is separate from the rest

What the pro does

  • Survey the lot, check setbacks and the existing foundation, and have an engineer or designer prepare drawings
  • Excavate and pour the new footing and foundation, and tie it to the existing structure as the plan requires
  • Frame the walls, floor and roof, and connect the new roof to the existing roof with proper flashing and drainage
  • Install the windows, doors and exterior cladding, and seal the connection between the old and new walls
  • Run new electrical, plumbing, heating and cooling into the addition, with rough-in inspections before the walls close
  • Insulate, drywall, finish the floors and trim, then complete the final inspections and walk through the room with you

What changes the price

Every job is different, so we do not publish prices. These are the things that move the quote up or down.

  • Site conditions, such as slope, soil type, drainage and access for the crew and equipment
  • Foundation type, which depends on the soil, frost depth and the connection to the existing house
  • Roof tie-in complexity, including the valley, the height difference and the drainage path
  • The size and number of windows and doors, and whether the exterior matches the existing house
  • New utility runs, the size of the heating and cooling system and any electrical service upgrade
  • Finish level for flooring, cabinets, bathrooms and the trim detail that matches the old house

What a written quote should spell out

  • Engineered drawings and the engineer's or designer's name and scope, listed with the permit package
  • Foundation type and depth, with the soil report or site assumption stated
  • Framing, roofing and flashing methods with the tie-in to the existing roof named
  • Window and door sizes and types, the cladding and the exterior finish details
  • Electrical, plumbing and HVAC scope, with the rough-in and final inspection steps listed

Questions to ask before you hire

  • Who designed the addition and who is the engineer of record for the foundation and framing?
  • What does the foundation look like, and how does it connect to the existing house?
  • How do you tie the new roof into the old roof, and how is water kept out of the joint?
  • What inspections does the job need, and who schedules them?
  • Who is the licensed electrical and plumbing contractor on the job?
  • Can you show me a home addition you completed with a similar foundation and roof tie-in?

Red flags

  • The pro starts without drawings or without an engineer's review of the foundation and framing
  • The pro says no permit is needed for a room that adds living space to the house
  • The quote does not say how the new roof connects to the old roof or how the joint is flashed
  • The pro wants most of the payment before the foundation is inspected

What you can do yourself

Clearing the site, moving landscaping out of the way and selecting finishes are reasonable homeowner tasks. Foundation work, framing, roofing, and any change to the structure or utilities must be designed and built by licensed professionals under permit, because errors in these areas are expensive and dangerous to fix later.

Licenses and permits

A home addition very often needs a building permit, engineered drawings, and licensed trades for electrical, plumbing and sometimes mechanical work. Check with your state or local licensing agency and your local building department before any excavation begins.

Home addition questions

How long does a home addition take?

Most of the time goes to design, permits and the foundation, and then to framing and the roof. Ask for a schedule with the inspection stages marked, and ask how weather and inspection timing are handled.

Do I need an architect or engineer?

Often, yes, for the drawings and the structural design. Ask the pro who is responsible for the structural calculations and whether the local building department requires a stamped set.

Will the addition match the existing house?

It can match in roof pitch, window style and siding, but matching may take special materials or a slightly different detail. Ask the pro to show the exterior elevations before you choose the materials.

About this guide

This national guide is written and checked by infinibell. Rules and conditions differ by place, so local pros are credited on guides for the cities they serve, not on national ones.

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