Mixed-Use Property: How to Split Costs Between Business and Home

What Is a Mixed-Use Property?

A mixed-use property is one used for both business and personal purposes. For sole traders and freelancers, this most commonly means running a business from a home you also live in — whether that's a dedicated studio, a converted garage, or simply a room used as an office. HMRC does not allow you to claim the full cost of running the property as a business expense. Instead, you must apportion costs on a fair and reasonable basis.

Why Apportionment Matters

Overclaiming expenses is one of the most common triggers for an HMRC enquiry into a sole trader's tax return. If you deduct more than the genuinely business-related share of your property costs, you risk penalties, interest on underpaid tax, and having to repay the excess. Underclaiming, on the other hand, means you pay more tax than necessary. A clear, documented apportionment method protects you either way.

Which Expenses Can Be Apportioned?

The following costs can typically be split between business and personal use:

• Rent — if you rent your home
• Mortgage interest — only the interest element, not capital repayments
• Council tax
• Utilities — gas, electricity, water
• Broadband and phone — where there is genuine business use
• Buildings and contents insurance
• Cleaning and repairs — for shared areas or the business space specifically

Costs that relate entirely to the business space — for example, repainting only your office — can usually be claimed in full without apportionment.

How to Calculate Your Business Proportion

HMRC accepts several methods, but the most common and straightforward is floor area. Divide the area used exclusively for business by the total floor area of the property, then apply that percentage to each shared expense.

For example: your home is 80 square metres. Your dedicated office takes up 16 square metres. That gives a business proportion of 20%. If your annual utility bill is £2,400, you can claim £480 as a business expense.

If a room is used for both business and personal purposes — say, a dining room you work in during the day — you should apply a further time-based split. If you use it for business roughly 40% of the time, your claimable share would be 20% (floor area) multiplied by 40% (time) = 8% of the total cost.

Always use the method that most accurately reflects actual business use. HMRC's test is simply that the split is just and reasonable.

The Simplified Expenses Alternative

If the calculation above feels complex, HMRC offers simplified expenses for sole traders. You claim a flat monthly rate based on the number of hours you work from home each month:

• 25 to 50 hours: £10 per month
• 51 to 100 hours: £18 per month
• 101 or more hours: £26 per month

Simplified expenses are easy to use but often produce a lower deduction than actual apportioned costs, especially if you have a dedicated business room or high property overheads. It is worth running both calculations before deciding which to use.

One Important Capital Gains Warning

If you own your home and claim a portion of it as exclusively for business use, you may lose the Private Residence Relief (PRR) on that proportion when you sell the property. PRR normally exempts your main home from Capital Gains Tax. Claiming exclusive business use of a room can create a taxable gain on that share of any profit. Many sole traders deliberately avoid claiming exclusive use for this reason, instead treating rooms as dual-purpose. Take advice before making this decision.

Keep Your Records

Whatever method you use, document it. Keep a record of your calculation, the floor plan or measurements, and all underlying bills. HMRC can ask you to justify your figures up to four years back — longer if errors appear careless or deliberate. A simple spreadsheet updated each tax year is all you need.