Home Addition Calculator: Estimate Your Cost To Build A Shed Dormer

Estimate how much it will cost to build a shed dormer addition in your unused attic. The quote includes the cost of cutting out a roof section, dormer framing, building a shed dormer, installing new windows, insulation, roof, and a bedroom + bath / shower room.

A shed dormer increases the usable living space in your house by an average of 200-250 sq. ft. This project's resale value is around 84.3%, making it a smart investment.

Addition Size: x Sq. Ft.
New Windows
New Roof Skylights
Bath / Shower type
Your Region:

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Mid Range
High End

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Example Of A Shed Dormer Addition

Attic Addition

Addition Calculator Features

To make this cost estimate as accurate as possible, the following set of specs is used:

A "Medium Quality" addition includes the following:

  • New Hardwood Floors in the bedroom
  • 5x8' Standard Bathroom With Shower, and Tiled Floor / Shower Surround
  • Spray Foam Insulated ceilings and walls, with a sheetrock / plaster finish
  • 4 new construction windows and 2 Velux skylights
  • Existing stairs leading up to the attic with new railings (does not include building new stairs)
  • New electrical wiring and and HVAC(Radiators or Air Ducts) installed
"Luxury Quality" makes use of higher quality, premium looking materials.

"Basic Quality" will have materials suitable for a Rental property. This includes: carpet floors, basic ceramic tile and an acrylic shower in the bathroom.

For a more accurate calculation, use the "Regional" setting to choose in which part of the US you live. Otherwise, use the "National Average". This unique feature allows you to get a truly custom estimate for your project.

Factors That Increase The Final Shed Dormer Cost

The total dormer cost can and will vary based on many factors.

If you live in a cold climate, the roof of your dormer addition will be susceptible to ice dams build up. This happens because in order to build it, you have to increase the ceiling height to at least 8 ft., making its roof almost flat.

This flat or low slope roof increases the chance of ice dam leaks. To avoid this problem, we recommend using a single ply membrane for low slope roofs, such as EPDM Rubber or PVC. This can increase your total cost by $1,400-2,500 on average. Use our Roofing Calculator to compare pricing for asphalt shingles, rubber or PVC single ply material.

Ins some cases, unexpected HVAC issues, such as difficulty in connecting existing system ducts / pipes to the attic, can also increase total cost.

Central Air HVAC Costs
Central AC only
$3900 - $5600
AC + Hot Air Furnace
$7900 - $10400
Central Air + Ducts
$12100 - $17900

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Design Considerations

Many homeowners stay away from building a shed dormer because they are often perceived as being ugly and ruining the overall curb appeal of the house. Unfortunately, there is some truth to this. A poorly designed and positioned dormer can indeed look like an eye sore on the roof.

However, if you plan everything correctly, it can actually enhance the beauty and functionality of your home. Consider the following tips:

- The dormer should look like its a section of the main roof. Build it with just enough space for windows and headroom to avoid having an oversized, protruding structure.

- Ideally, the structure should spring from a line that is below the primary roof ridge.

- The roof slope of your dormer should be minimum 4-in-12. If its flatter, the wall of the dormer will appear disproportionately tall, creating a highly unbalanced design.

- It is best to avoid building something than runs the whole length of the house. These types of additions look strange, because they are mistaken for being the main roof. Instead, nest the dormer within the eaves of the main roof, so it clearly looks like a subordinate structure.

- Contact 3-4 local pros to get free estimates and design suggestions. This way you will be able to choose the best price and a contractor who you feel comfortable entrusting such a huge project.

Cost to build 420 sq. ft. Addition:
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Mid Range
High End

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3 thoughts on “Home Addition Calculator: Estimate Your Cost To Build A Shed Dormer

  1. Alexander

    I am interested in a shed dormer on both sides of the attic of a classic cape cod to out a master bedroom with bath, and some extra storage space in.

  2. Joan

    I am trying to find out the cost of a shed dormer for our Cape house. We have two bedrooms in the attic and would like to add a bathroom.