Home tax basis, explained

Most homeowners meet the word “basis” once, in the week they sell, from an accountant asking for paperwork they don’t have. It is worth meeting it earlier, because almost everything that makes it useful happens years before the sale.

What basis actually is

Your cost basisis what the home cost you for tax purposes. It usually starts with the price you paid. Certain costs of buying can be added to it as well — the IRS calls these settlement or closing costs, and only some of them qualify (Publication 530).

From there it moves. Money you put into the home in ways that qualify may increase it. Certain events decrease it. The figure at the end — after everything that raised or lowered it — is your adjusted basis.

Why it matters when you sell

When you sell, your gain is broadly the sale price minus your adjusted basis. A higher adjusted basis therefore means a smaller gain. A smaller gain can mean less tax — though for many sellers it means no difference at all, and it is worth being honest about why.

If you meet the ownership and use tests, you may be able to exclude up to $250,000 of gain from your income, or up to $500,000 filing jointly (Publication 523). Many sellers fall entirely inside that exclusion and owe nothing on the gain regardless of their basis. Records still matter — you cannot know which side of the line you are on without them, and the exclusion has conditions.

What can raise it

Improvements that add value, prolong the home’s useful life, or adapt it to new uses may be added to basis. A new roof, an addition, a replaced heating system, a finished basement.

Ordinary repairs that simply keep the home in good condition generally are not added. This distinction does more work than any other in the whole subject, and it has real gray areas — it gets its own article.

What can lower it

This is the half most people never hear about, and it is the reason “price plus improvements” is an incomplete description:

• Insurance payouts for casualty losses, and casualty losses you deducted
• Certain energy credits and subsidies you received for the work
• Depreciation, if any part of the home was used for business or let
• Improvements that are no longer part of the home — the kitchen you fitted in 2011 and replaced in 2023 generally drops out (Publication 530)

That last one surprises people. Replacing an improvement does not stack two improvements onto your basis; the one that is gone generally goes with it. It is another reason a record with dates on it beats a shoebox.

Why this is a recordkeeping problem

None of the arithmetic is hard. What is hard is proving it a decade later. The invoice for a 2014 roof, the contractor’s statement for the 2019 kitchen, the settlement statement from the day you closed — those are what turn a number into a supported number.

An improvement you cannot document is an adjustment you cannot support. The IRS is explicit that you should keep records that support the basis of the property (Publication 551), and the practical version of that advice is simply: write it down while the paper is still in your hand.

Where to start

If you have just bought: keep the settlement statement, and log the purchase price and date. That is your starting basis, and it takes two minutes.

If you have owned for years: start with what you can still evidence. A partial record built today is worth more than a complete one you meant to build and never did.