Say you've just won a bidding war on a 1962 ranch on a 15,000-square-foot lot in Wellesley. The number that got you there was the sale price. The number that decides what you can actually build there has nothing to do with what you paid, what your architect drew, or what your construction loan approves. It's a formula the town calculates before you own the lot, and it will cap your house whether your budget is $2 million or $6 million.
That formula has a name almost no one outside a Wellesley building permit application has heard: TLAG, or Total Living Area plus Garage. It is the mechanism behind the town's Large House Review process, and it explains why two buyers with identical budgets on different Wellesley lots can end up building very different houses.
The Formula Nobody Puts in the Listing
Wellesley adopted Large House Review to slow what town planners once called "mansionization," the trend of new construction and additions growing well past the scale of surrounding homes. The town's own description of the program states its purpose is to minimize the impacts of large houses on their surroundings, addressing landscape preservation, building scale, lighting, open space, drainage, and circulation.
The mechanism is a threshold. If a proposed new home's TLAG exceeds the limit set for its zoning district, the project cannot get a building permit until the Planning Board and Design Review Board sign off. The same rule applies to additions that push an existing home over the line, or that grow TLAG by 10 percent or more on a home already above it.
What Your District Actually Allows
The threshold is not one number for the whole town. It scales with the minimum lot size of the zoning district, and it is public record.
| Zoning District (typical minimum lot size) | TLAG Threshold |
|---|---|
| 10,000 sq. ft. | 3,600 sq. ft. |
| 15,000 sq. ft. | 4,300 sq. ft. |
| 20,000 sq. ft. | 5,900 sq. ft. |
| 30,000 / 40,000 sq. ft. | 7,200 sq. ft. |
Locally these districts get shorthand names like SR10, SR15, and SR20, and the pricing tends to sort along the same lines. New construction on an SR10 lot generally lands in the $2.5 million to $3.3 million range. Move up to SR15 and builds commonly run $3.3 million to $4.2 million. On SR20 or larger, new construction regularly starts above $4.2 million. The lot size buys the ceiling. The sale price just buys the lot.
The Square Footage You Didn't Know You Were Building
TLAG is not a casual estimate. It is calculated from a formal affidavit, and it counts space most buyers would not think to include. A finished attic counts. A garage counts in full, even a three-car garage attached to a modest house. Closets count. Basements count too, but only proportionally, based on how much of the foundation wall sits above grade.
The town's own guidance walks through the math: if a basement wall averages 2.5 feet above grade out of an 8-foot total wall height, that's a ratio of roughly 31 percent, and roughly 31 percent of that basement's square footage counts toward TLAG. Drop below a 25 percent ratio and none of it counts. It's a threshold within a threshold, and it's the kind of detail a 2010 Wellesley Patch report on the finalized rules spelled out specifically so buyers wouldn't try to hide square footage in basements, garages, or attics.
Space that commonly gets counted toward TLAG, whether or not it shows up on a real estate listing:
- Finished or unfinished garages, at full footprint
- Attics, finished or not
- Closets and other enclosed storage
- Basement area, prorated by the wall-height ratio described above
- Any addition that grows the existing footprint by 10 percent or more once the home is already near the threshold
The One-Family Exemption Nobody Advertises
Wellesley has a second, larger-scale review called Project of Significant Impact, or PSI, and it's worth knowing what it does and doesn't touch. PSI applies to construction projects other than one-family or two-family dwellings, once new floor area reaches 10,000 square feet or more, or renovated and replacement floor area reaches 15,000 square feet or more in a building with at least 15,000 square feet of ground coverage. It exists to evaluate off-site impacts on water, sewer, drainage, electric capacity, fire protection, and traffic.
Active 2026 dockets show the process at work. Proposals at 592 Washington Street and 49 Walnut Street both moved through PSI review this year, each requiring months of engineering analysis and a sign-off from the Municipal Light Plant confirming the electrical grid could handle the load before either project could advance. If you're buying a single-family teardown lot, none of that applies to you directly. Your house, no matter how large, is governed by Large House Review, not PSI. The distinction matters because it tells you which board you'll actually be sitting in front of, and PSI's multi-department review process is considerably longer and more involved than LHR's.
Reading the Market Through the TLAG Lens
None of this happens in a vacuum separate from price. Wellesley's average home value reached just over $2.07 million as of May 2026, and separate market tracking as of March 2026 showed average days on market stretching from 18 days a year earlier to 76 days, a real signal that the frantic pace of recent years has eased somewhat even as prices hold near record levels.
The nonprofit Building a Better Wellesley put the town's median home value at $2.04 million as of February 2026, and cited a 2020 Brookings Institution estimate that building a new single-family home in Wellesley Hills costs an average of $1.9 million, compared to roughly $500,000 per unit for new condominium construction. That gap is the clearest evidence of what buyers are actually paying for in this town. It is not the existing structure. In most teardown scenarios that structure has effectively zero value once demolition costs are factored in. What buyers are pricing is the land, the zoning district it sits in, and the TLAG ceiling that comes attached to it.
Before You Make an Offer
If you're considering a Wellesley property with teardown or major addition potential, a few steps can save months later.
- Confirm the zoning district and lot square footage through the Assessor's records before you write an offer, not after.
- Run a rough TLAG estimate against your target house size, including garage and any finished basement or attic space you're planning.
- If your number lands close to the threshold for that district, build the 3 to 4 month Large House Review timeline into your closing and construction schedule rather than treating it as a contingency.
- Ask whether the existing structure is legally nonconforming. In some cases that status routes a project to the Zoning Board of Appeals instead of Large House Review, which changes the process entirely.
A Few Questions Worth Asking Directly
Does Large House Review apply to a home that's already over the threshold? No. The review only applies to new construction and to additions. An older home that was legally built above today's TLAG threshold does not need to go through the process unless you add on and cross the 10 percent growth trigger.
Can a nonconforming lot get me out of Large House Review? Sometimes. Homes that require a Zoning Board of Appeals special permit for a nonconforming structure or lot are exempt from LHR, though that simply moves your project to a different board with its own review standards.
Is Large House Review the same thing as the Project of Significant Impact permit? No. PSI is reserved for projects other than one-family and two-family homes, once new construction reaches 10,000 square feet or more. A single-family teardown, however large, is reviewed under Large House Review, not PSI.
Wellesley's zoning rewards buyers who do this math before they fall in love with a lot, not after. If you're weighing a teardown or a major addition anywhere in town, The Bauman Group can walk the TLAG numbers with you against a specific address before you write an offer, so the house you're picturing and the house you're allowed to build are the same one.
