BuyingReal Estate April 16, 2019


You love the neighborhood!  You love the lot!  You would love the house, but just with a little more space!  Don’t just assume you can expand.  It is time to pause.  While expanding your home in most instances can be accomplished, you need to do a little more research about the process to verify you can in fact expand to make your dream home a reality.

If the home you are looking at is on municipal sewer, generally you can expand.  You should confer with the local Building Department on set backs and other requirements.  If the lot is non-conforming or near wetlands, there may be additional steps.

If the home you are considering is on private septic, you may likely be able to expand, but there are several more factors that may impact your ability to expand and how large of an addition you can build.  The factors can vary by Town so this list is not exhaustive, but representative of some main items to consider to gain approval to expand a residential property.

Adding a Bedroom – If you are adding a bedroom you will likely need to upgrade your septic in most instances.  Most septic systems are rated for a certain number of bedrooms (not bathrooms).  If you have a three-bedroom house with a three bedroom septic, adding a fourth bedroom will require a new four bedroom septic system.  But upgrades to septic systems are often treated like new construction, so depending on the size of the lot, upgrading to a four-bedroom septic may not even be possible since you need space both for a leech field and a reserve leech field.  A new septic system can easily cost $25,000+.  If all that may seem mind numbing, your Buyers Agent can explain it in more detail, or better yet, speak with the local Board of Health – the experts in such matters.

Adding Square Footage – If you are adding a certain amount of square footage (say, 59% or greater to the existing house) you may also be required to upgrade the size of your septic system.  Which depending on your lot size you may or may not be able to do.  The huge caveat is that the Town may include in that percentage % calculation “any” space added since the house was “originally” built….so that finished basement from the 1970s may need to be included in that calculation further limiting the size of the expansion before a new septic system is required.

Adding Several Rooms – If you are adding a certain number of rooms – that may also require a septic upgrade.  Some towns take the number of rooms, divide by 2 and round down.  So that eight room, four-bedroom home, with a four-bedroom septic gets the following calculation (eight rooms/2 = 4.0).  You have a four-bedroom septic so you are fine.  Add one room (nine rooms/2 = 4.5, round down to 4.0 you are fine.  Add two rooms (10 rooms / 2 =5, you have a four bedrooms septic, and you may need to upgrade to a five bedroom septic.

What does all this mean for the Home Buyer?  Don’t assume you easily expand any home.  ‘

You need to have conversations (preferably in-person) with the local Board of Health and Building Department (or other parties).  Add in the Conservation Department if you are near anything resembles water – meaning actual water, wetlands, seasonally wet, etc.  This can be near impossible to do in the heat of a multiple offer situation.  If you have your two or three Towns narrowed down, it might be prudent to have preliminary conversations to understand how each Town approaches this.  In the Greater Boston area we have an older housing stock, which means many homes are smaller than Buyers will eventually want or will eventually need.

Doug McNeilly is a REALTOR® with Coldwell Banker Residential Brokerage in Wayland, Massachusetts.  He specializes in Wayland, Sudbury and the Greater Boston Metro West Area.  He can be reached at or

A modern home with an extension being built