How to calculate a special assessment per unit.

The formula is a document question before it's a math question

The calculation starts with the declaration and the applicable statute, not an assumed choice between equal shares and square footage. The authorized formula could be equal shares, recorded common-expense or ownership percentages, class allocations, benefit-based allocations tied to what the project serves, or another formula the governing documents specify. Raw square footage should not be used as a substitute for recorded allocated interests unless the governing source actually makes square footage the formula. Language like "common expenses shall be shared in proportion to each unit's percentage of undivided interest" locks the association into that method regardless of whether the board thinks equal share would be simpler or fairer, and applying the wrong method can void the entire assessment — so confirming the specified formula is the first step, not an afterthought.

Equal share method, worked example

Equal share divides the total project cost by the number of units, with every owner paying the identical dollar amount regardless of unit size: Total Project Cost ÷ Number of Units = Assessment per Unit. For a parking lot repaving project costing $48,000 across a 60-unit association, each owner pays $800 — the same figure whether their unit is 700 square feet or 1,800 square feet. Associations with largely uniform unit sizes, or HOAs governing detached single-family homes on similarly sized lots, most often use this method because it's simple to communicate and audit.

Square-footage (proportional) method, worked example

The proportional method calculates each owner's share based on their unit's percentage of the total assessable square footage across the community: (Unit Square Footage ÷ Total Square Footage of All Units) × Total Project Cost = Assessment per Unit. In a mixed-size condominium, a 1,200-square-foot unit might carry a 3.8% ownership share while a 900-square-foot unit carries 2.9% — so on that same $48,000 project, the larger unit owner would owe roughly $1,824 while the smaller unit owner would owe roughly $1,392. This method is standard in most condominium declarations, where the recorded percentage of ownership interest already exists for exactly this purpose.

Where boards get this wrong

The most common error is defaulting to whichever method feels intuitively fair — often equal share, because it's simpler to explain at a meeting — without actually checking what the governing documents specify. The second most common error is applying square footage using unverified, informally remembered unit sizes rather than the recorded figures in the condo plat or the county assessor's records, which can produce an allocation an owner successfully challenges later if the underlying numbers don't match the recorded documents.

Mixed-use and unequal-amenity complications

Some governing documents split allocation differently depending on what the project actually benefits — a roof repair might be allocated only among the building's units under that roof rather than the entire association, while a shared amenity like a pool might allocate by a separate formula written specifically for amenity-related expenses. A board should confirm whether the specific project falls under the general common-expense formula or a narrower allocation clause tied to that particular asset before running any calculation.

Documenting the calculation for the record

• Cite the specific CC&R or Declaration section authorizing the allocation method used.
• Show the total project cost, the allocation basis (unit count or square footage table), and the resulting per-unit figures in the board minutes or a published calculation sheet.
• Attach the vendor bid or contract that produced the total project cost.
• Keep the calculation sheet with the special assessment notice sent to owners — it's the first document an owner or attorney will ask for if the assessment is challenged.

Before the notice goes out

Confirm the board-vote and/or membership-vote threshold the governing documents require for a special assessment of this size, and confirm the notice content and timing separately from the calculation itself — a mathematically correct assessment sent with a defective notice is just as vulnerable to challenge as an incorrectly calculated one sent with a proper notice.

Payment plan and hardship considerations

Even a correctly calculated assessment can create real hardship for individual owners, particularly on a large capital project. Many governing documents and state statutes permit — or in some cases require — the board to offer a payment plan option for special assessments above a certain size, spreading the amount over several months rather than demanding a lump sum. Boards should decide and document the payment plan terms (length, interest if any, and what happens on default) before the notice goes out, not reactively after the first hardship request arrives, since an ad hoc payment plan offered to one owner but not another can itself become a fairness challenge.

How a miscalculated assessment typically gets challenged

An owner who believes their allocation was calculated incorrectly — using the wrong method, an outdated square-footage figure, or a total project cost that wasn't properly documented — most commonly raises the issue first informally with the board or manager, then formally through the association's internal dispute process if one exists, and as a last resort through litigation or, in some states, a mandatory pre-litigation mediation process. Associations that keep clean documentation of the method, the source figures, and the vote can usually resolve these challenges quickly; associations that can't produce that paper trail face a materially harder and more expensive dispute.

It's also worth confirming, before the notice goes out, whether any unit type in the community — a commercial unit within an otherwise residential condo, or a unit with a modified use restriction — is subject to a different allocation percentage under the governing documents, since a single flat calculation applied uniformly across a community with mixed unit types is a common source of downstream disputes that a closer read of the Declaration would have caught early.