About this plan

A 20 foot square cottage ADU, 400 square feet over two levels with one bedroom and one bathroom, under a shallow 3 in 12 shed roof. You receive these 400 sq ft adu plans as a PDF plan set.

Four hundred square feet on purpose

400 square feet is a threshold number. It sits under the floor area cap that many jurisdictions apply to accessory dwellings, and under the height limits too, with the ridge kept to 15 feet 10 inches. If you are building in a place with tight ADU rules, this plan is drawn to fit inside them rather than to argue with them.

The porch is a third of the building

There is 140 square feet of covered porch against 400 square feet of interior. That ratio is unusual and it is the reason the cottage does not feel cramped: for much of the year the porch is simply another room, and it takes the boots, the chairs and the firewood that would otherwise be indoors.

Inside

Ceilings run from eight feet at the low wall to thirteen at the high one. There is proper washer and dryer space rather than an assumption you will use a launderette.

Framing and foundation

Light timber framing throughout, which keeps the material list short and the build within reach of a competent DIY builder. Slab, pier and crawl space foundations are all drawn.

What you get

A complete construction set, supplied as a PDF:

• Dimensioned floor plans for each level, drawn to scale
• Exterior elevations on all four sides
• Building sections and construction details
• Foundation plan with floor, wall and roof framing layouts
• Electrical layout
• Window and door schedules
• Roof plan with pitch and overhang dimensions

Before you build

Accessory dwelling rules vary more than almost any other part of the residential code. Some jurisdictions permit them outright, some cap the floor area, some require a separate utility connection or an extra parking space. Check what your county allows before you order materials. The rules have been loosening across much of the country, so if you were told no a few years ago it is worth asking again.

Code requirements are set locally. The International Code Council publishes the model codes most United States jurisdictions adopt.

Other plans to compare

If these 400 sq ft adu plans are not quite the right size, browse every design in Cottage, or compare this one with American Cottage, 30×30 and Spruce Cabin, 34×36.

Common questions

Can I modify these 400 sq ft adu plans?

Often, yes. Common requests include moving a wall, mirroring the layout or adapting the foundation to your site, but what is possible depends on the plan, so ask for a free quote before you buy. Editable CAD files are not available for every plan; email us to confirm availability for this one.

How are the drawings supplied?

Plans are supplied digitally as PDF files. Our Shipping and Delivery Policy explains how and when files are made available.

Are there limits on printing?

No. Print whatever your builder, your subcontractors and your permit submission need.