The Math of Your Home Size. Part 2: Your Guide to Measuring and Expanding Your Home.

You may want to start with Part 1: Make the Best of the Space You Have.

Ready to follow your expansion dreams? Here is your guide on what you can do. The guidelines vary from town to town, and could be adjusted at any time, but you need to know the basics in order to adapt to these changes and know what you can ask for. Sit together with an architect, and play with some math to see what space you need, how much you could spend, and whether your goal is to increase the official home living area. Then, you might consider consulting with an experienced real estate agent or appraiser to see what your home’s increased value would be after the expansion.

In order to check your expansion options, you need to start by figuring out your current home size. It’s like the starting point of a dieting journey, except that here we hope to gain rather than lose. How do you measure it? Pretty intuitively: take your home plan if you happen to have it (such as the one displayed below) or create one by drawing a bird's eye view of your home. Go around the wall perimeter, measuring and marking every wall. For homes you usually measure around exterior walls, for condos – around the interior. Adjust the shape if necessary, then split it into rectangles, compute the area of every rectangle (length of one side times the length of the other), add it all up, and voilà. But, wait! Do you count the bathrooms? What about the staircase? And the back porch that you just turned into a sunroom? Is it counted as part of your square footage?

These mystical GLA initials next to the square footage in your mortgage papers or appraisal report, as it turns out, mean Gross Living Area. The GLA is defined as the total area of finished, above-ground, residential space, excluding unheated areas such as porches and balconies. See the sample home (Dover design) and the floor plans from the CadSmith Studio LLC





All the areas highlighted in pink and blue are counted as part of the GLA.

1st floor: 28' x 38' + 16' x 24' = 1064 sq ft + 384 sq ft = 1448 sq ft
2nd floor: 28' x 38' = 1064 sq ft

Total GLA: 1448 sq ft + 1064 sq ft = 2512 sq ft

Here are home areas in question that are counted as part of the GLA:

• bathrooms are in


• halls are counted


• closets are in


• even the dead space inside stud walls is calculated as living space (except for the eaves in cape style and some other styles of homes)


• staircase area is counted twice – once for each floor


• porch turned into a sunroom is counted if heated and used year around


• high-ceiling finished and heated attic is counted

Here is a list of areas NOT counted in the GLA:

• open foyer -- essentially a two-story hole that is not counted twice, but just once


• porches and balconies (unless heated and used year around)


• finished basement – surprise, surprise - is out of the GLA unless it is completely above ground level and used year round; however, an appraiser will usually include them in a separate category of “basements and finished areas below grade.”


• low-ceiling attic (see details below)

Ok, we know what we've got now, the current GLA. Ready to expand?

First, beware: increasing your GLA will likely lead to greater home value but also will increase your taxes.

According to broker and appraiser Sam Schneiderman, President of Greater Boston Home Team, the least expensive way to create an extra room or two of living space is usually to finish your attic.

Will a finished attic increase your GLA? Yes, if you can reach it from inside your home, without sneaking through the garage or romantically ascending exterior stairs, and if it fits your town's legal room height requirements. In some neighborhoods, for instance, the portion of the attic that has at least 5' height and an average ceiling of 7' is counted as legal GLA. Check your numbers carefully with your local building inspector and appraiser, keep it legal, especially if you plan on placing your mother-in-law or live-in nanny there.

If your attic is too small or non-existent, consider insulating and heating your garage or basement. Remember that the basement most probably won't add to the official GLA numbers, unless it is completely above ground. But then do you call it a basement? :)

If you do decide to expand, then you have to know a bit of expansion math. Luckily, it is quite logical. Unluckily, each town has its own rules and may have their own methodology when it comes to calculations. The most common rules according to Mr. Schneiderman are:

Setback Rules: To keep peace with neighbors, guard privacy, and avoid fire hazards, town building codes require some minimum space between your house and your neighbor’s house. They are called setbacks and vary at the front, side and back of your lot. Setback space is off limits for construction. You can find setback requirements in your town's building code. Then, take your lot plan and mark these “buffer” areas off-limits for any new construction. (If you can only expand by encroaching into the setback areas, ask the town building inspector about the likelihood of applying for and receiving a variance.)

Max Lot Coverage Rules: Still have some space left on the lot plan to expand sideways or up? Check your Max Lot Coverage % in your zoning district. There could be a limit on how much of your lot can be covered by your house. If applicable, maximum coverage rules are calculated as follows:

House Footprint / Lot Size <= Max Lot Coverage Therefore, you can expand as long as: (House Footprint + addition) / Lot Size <= Max Lot Coverage Your house footprint is just what it sounds like – square footage of the print your house makes on the lot.

Floor Area Ratio (FAR) rules: Most towns usually limit how much living area is allowed on lot of a given size. The limits can vary depending on the neighborhood or zoning district. Check the Floor Area Ratio (FAR) to see if you can expand your GLA. If applicable, the FAR rule is typically:

GLA / Lot Size = FAR

Therefore, as long as:
(current GLA + addition) / Lot Size <= Maximum FAR you can expand without a variance

Maximum Building Height Rules: As you may have suspected, you can't build an Empire State Building on your lot (because town fire equipment may not have Superman on staff to reach the top if it catches fire!) Most areas have a maximum height limit on buildings. Again, your town's building code is your bible. In my neighborhood it is 2.5 stories and 35'.

So, learn your expansion math dance with your architect. Ask your town's building inspector to approve your steps (calculations). Only after that, hire your contractor partner and get started with measured and riveting twirls. It all requires team work. Don't skip your steps.

While it may seem like a lot to consider, the math of it all is elementary. Beware that rules vary from town to town, and are updated from time to time. Your bank appraiser's calculations may disagree with the town building inspector's, but now you know what they are talking about and what you should be asking for. Enjoy playing with these numbers to plan, dream and negotiate your home's expansion to meet your needs and realize your dreams.

Meredith and Derek dream up the plan for their house on Grey's Anatomy

P.S. I would like to thank Sam Schneiderman of Greater Boston Home Team, Bonny Smith of CadSmith Studio LLC, Needham (MA) building inspectors as well as Anat Yosfan of AnatYDesign for their help in preparing this article.