Home Addition on Slab or Basement

Summary: If your lot is flat, and all other things are equal, it is generally less expensive (cheaper) to build above grade and to build with wood than it is to build below grade and to build with concrete or masonry.

Hello,

We are considering adding on to an existing house using a slab
foundation.

We live in MO and it is very rocky.

What I want to know is, would it be cheaper to have a basement with one story or have a slab and have two stories?

Thanks,
Renee

Hi Renee,

If your lot is flat, and all other things are equal, it is generally less expensive (cheaper) to build above grade and to build with wood than it is to build below grade and to build with concrete or masonry.

If your lot is sloped, a crawl space would be the next least expensive or you could even entertain digging into the slope to obtain a level area for a slab.

An alternative I have used is to level the actual building site (dig into the slope), build a retaining wall (with a drainage system), and build the house on a slab foundation on the resulting flat area.

The retaining wall could be made from wood, railroad ties, stone, block, brick, etc., or even an earth berm made from your own dirt.

There are always trade-offs in any construction decisions and with your project you can see you will have a few.

Good luck,
Carl

Adding a Second Story to an Existing House

Carl,

We would like to add a second story to our 1963, 1500 sq ft ranch home using the same footprint (foundation).

Besides the addition, we want to change the layout of the first floor from 4 bedrooms, kitchen, and family room, and one full bath to an open 2 story foyer, bigger kitchen, master bedroom with a full bath, bigger family room, and a powder room.

The new second story addition needs to have three decent size bedrooms.

One bedroom needs to have an attached bath and other two can share a bath.

Do you have any suggestions, floor plans, approx. cost etc.?

We do not have an Home Owners Association (HOA) to contend with.

Thanking you in anticipation that you will help,

Raj

Hi Raj,

I’m afraid that what you are proposing is much more than an addition.

What you really are saying is that you want is a new 2 story home built on your existing foundation.

I don’t want to get into discussing structural framing requirements but even the existing 1st floor will have to totally redone as load bearing walls for the 2nd floor will be needed. This means that even the basement will have to be prepared for these loads.

The only savings you might realize would be utilizing the existing the existing foundation walls if they are structurally sound.

Since the house was built in 1963 I doubt that even the existing windows are worth keeping.

To get a (very) rough estimate of the cost of doing this I would run a 3,000 sq ft house through the “cost to build calculator" found on my getting started page.

I would use the lowest quality class (#6) for all categories and ignore any other questions that don’t apply as you go through the estimating process.

I believe that even the reworking of the basement support columns (or beams) will be included doing this estimate as well as the total rebuilding of the 1st floor the 2nd floor addition.

This rough estimate will give you an idea of what this project could cost

Check with your local Building Inspection Department to find out if there will be any issues or problems increasing the size of your house as far as they or any of the other departments (zoning, health, etc.) are concerned.

Obtain the services of a local home designer or architect to look the situation over and work with you on designing what you want.

At the same time, find a good local home builder (General Contractor) and get some ballpark estimates on a “turn key” fixed price contract job…NO cost plus estimates or contracts.

A good home builder can also help you with the design process along with the home designer or architect.

Be aware that you may be overbuilding for the area and therefore not receive enough of an increase in your home’s value to compensate for the cost of the project.

You obviously will not be able to live in this structure during its reconstruction so factor in the cost of a rental home and moving twice along with the construction costs.

It’s a big project, one I would not recommend you try to contract yourself.

Good luck,
Carl