Splitting the running costs when part of your home is used for a business

One room is the office, the workshop or the stockroom — and the bills for electricity, heating, water, refuse and insurance still arrive for the whole home. Somebody has to say which part of that falls on the business. That is a division by floor area, and it is exactly what CostDivider does: one group per year, the business as a member holding the area share, every bill recorded once — and at the end of the year there is a figure you can hand to your bookkeeper or tax adviser.

To avoid any misunderstanding up front: CostDivider is not a tax program. It does not know what is deductible where you live, which areas count as business use, what evidence is required, or how any of it belongs on a tax return — and it produces no document for a tax office. It divides costs by the key you give it, and works that key through reliably. Whether the share is deductible, and how much of it, is a question for your tax adviser. The arithmetic underneath is what this app does.

In short

1. Measure: the total floor area, and the area used for the business.
2. One group per year, for example "Home costs 2026".
3. Two members — "Private" and "Business" — with the square metres as percentages.
4. Only costs that genuinely follow the floor area go into the group.
5. Record every bill as an expense, with the party that actually paid it as the payer.
6. Lock at the end of the year — the business's share is the figure you hand on.

The key is floor area

You can work directly in square metres: with 100 m² of living space of which 18 m² is the office, you enter 18 for the business and 82 for private use. CostDivider balances the values so that together they cover the full cost — they do not have to be brought to 100. You could just as well think of 18 and 82 as percentages; here they are the same numbers, and square metres are the version you can check against a floor plan later.

Which areas count is not the app's decision. Whether only the room itself counts, whether hallway and bathroom come in proportionally, whether a room has to be used exclusively for the business — that is the question your tax adviser answers, and it is answered differently in every country. CostDivider takes the number you enter. Once it is settled you enter it once, and it applies to every bill in the group.

The business is an ordinary member

This is the leap the guide is built on: a member in CostDivider needs nothing but a name and a share. No email address, no account, no invitation. "Business" is a name like any other — you add it the way you would add a person, and give it the area share.

From then on the settlement works out by itself what you would otherwise calculate by hand on every bill. In the owed share column, "Business" carries the amount falling on the business area — for the year as a whole and, if you open a single expense, for each bill inside it.

Only the costs that follow the area

A share always applies to the group's total, never to a single expense.

So only items genuinely divided by floor area belong in this group. A purely private bill in there would hand 18% of itself to the business; a purely business purchase would send 82% of itself into the private side. Neither is conspicuous — there is a plausible-looking figure either way.

What belongs in is whatever arises for the home as a whole:

• Heating, electricity, water and waste water, where they are not metered separately.
• Refuse, street cleaning, chimney sweep, caretaker, maintenance.
• Buildings insurance, property tax, service charge.
• The rent, if you want to divide it by the same key.

What stays out is anything belonging wholly to one side. Purely business costs need no division at all — they are business costs in full and belong in your books, not in this group. Purely private ones just as little.

When a room has a meter of its own

Measured beats apportioned. If the workshop has its own electricity meter, its consumption is no longer an estimate but a reading — and a reading is a fixed amount in CostDivider, not a percentage.

Fixed amounts are taken off first; whatever is left is divided among the percentage members. One member therefore cannot have both at once in the same group — either a fixed amount or a percentage. If you need both keys, the clean answer is two groups:

• "Home costs 2026 – floor area" with the percentages: heating, insurance, refuse, property tax.
• "Home costs 2026 – metered" with fixed amounts: the workshop's metered electricity.

One year, one group

The reason is the same as in any utilities settlement: the settlement knows no periods of time. A share is computed from the group's total, not from a given date onwards.

Does the area change mid-year — a second room joins, a room goes back to private use? Then you settle at that point, lock the group, and start a new one on the new key: "Home costs 2026, from 07/2026". At the end of the year you add the two shares together. Two groups for one year are no problem; one group holding two truths is.

As a by-product, a locked group is exactly what you go looking for three years later when somebody wants to know how the figure came about: a closed position with every single bill still in it.

What stands there at the end of the year

The settlement shows three figures for "Business": the owed share — what falls on the business area — the paid in, normally zero because the private side paid, and the balance. The owed share is the figure this is all for. In the example below it is 572.40 out of 3,180.00 in total costs.

The settlement suggestion additionally says "Business pays Private 572.40". Whether that money really moves from a business account to a private one depends on how you keep your books — the figure is the same either way.

When other people live there too

If you do not live alone, nothing about the set-up changes; there are simply more members. With 100 m², of which 18 m² is business use, and two people on the private side: "Business" gets 18, the two of them 41 each. CostDivider balances the values, and the business share comes out exactly as it does in the example below.

That is precisely why the business works as a member: a share is a weight, and the arithmetic is indifferent to who or what stands behind it.

Receipts stay with you

What is stored are amounts, dates and descriptions — not receipt documents. For evidence you need the bill itself; the group is the account of the division, not the archive. Careful descriptions therefore pay off here more than anywhere else: "Electricity instalment 03/2026" can be found again, "Electricity" cannot.

Five moves that make the figure hold up

• Establish the area once, properly, and note how you got there — in the group name or in the description of the first expense. What gets questioned later is the derivation, not the result.
• Only area-dependent costs in the group. Anything else distorts every share in it, without ever looking wrong.
• Set your own share. Whoever creates the group is automatically a member with a fixed share of 0 — leave it there and the business carries everything. Change your own membership to the private percentage before you enter the first bill.
• Metered figures as fixed amounts, and in a group of their own. A meter is more accurate than any key — and that advantage is lost the moment the amount lands in a percentage group.
• Enter as payer whoever actually paid — not whoever received the bill. That is what the balance is built from.

Worth knowing

• Not a tax program. No VAT, no depreciation, no forms, no check of whether an item may be claimed at all. CostDivider divides and calculates — the tax judgement comes from your adviser.
• Percentages do not have to add up to 100. 18 and 82 work exactly as 18 and 41 and 41 do; the values are weighted against each other.
• A member has one kind of share per group — a percentage or a fixed amount, never both.
• The share link shows everything in the group to anybody holding it. For a group containing your household costs it is rarely what you want — your tax adviser gets the figure, not the link.
• Meter readings are worked out elsewhere and the result entered as an amount. That keeps every reading method possible.

Common questions

Does the business need an account of its own?

No. A member needs only a name, and a business has one. No email address, no invitation, no second login.

Can the rent go into the same group?

Yes, if you divide it by the same area key: then it is an expense like any other. If you want it shown separately, use a second group with the same shares — the division is identical, the total stands on its own.

What if the area share changes during the year?

Lock the group and start a new one on the new key. A share always applies to the entire cost of its group and not from a given date — two groups model a year like that correctly, one group holding two truths does not.

Does CostDivider work out the deductible amount?

It works out the area share. Whether that share is deductible, in full or in part, and what your country asks for in return, is what your tax adviser tells you — the app does not know the rule and does not pretend to.

Example figures. Green means the group owes that person money; red means they owe the group.

Next

• Settling up utility costs for a house or flat
• Settling up the flatshare
• Settling up a shared move-in budget
• Settling up a shared holiday
• Settling up a shared gift
• Collecting money for one bill, with everybody who joins sharing it