Custom Bill Split Rules: A Written-Rule Approach

Four flatmates share an apartment. Heating runs all winter and one bedroom is twice the size of the others. The internet is flat-rate. Water is small but everyone notices when one flatmate showers twice a day. One person travels every other week for work, another is on a study exchange for a semester. Equal split feels wrong, per-square-meter feels wrong for water, per-day feels wrong for internet. So what do you actually do?

A custom split with written rules means the flatmates agree at handover on a one-page rule sheet that specifies how each utility is divided — equal for some bills, per square meter for others, per day of occupancy for the rest — and then apply that sheet unchanged for the duration of the tenancy. It is the fourth and most flexible of the common methods, the one that gets you closest to “actually fair” in flats where no single rule fits every bill, and the one that requires the most upfront conversation in exchange.

Quick context first: this is a deep-dive companion to the broader piece comparing all four splitting methods. If you have not picked a method yet, start with Splitting utility bills among flatmates: 4 methods compared and come back here for the custom-rule details.

What a custom rule sheet actually is

A custom rule sheet is a written document, ideally one page, that lists every recurring expense in the flat and specifies the split method for each one. It is signed by every flatmate at handover, stored in shared documents, and applied unchanged unless the group formally updates it.

The structure that works in practice has three columns: the bill, the method, and a short rationale. The rationale matters more than people expect — when an edge case arrives six months later (a new appliance, a long-stay guest, an unexpected charge), the “why” of each rule is what tells you how to handle the new situation without inventing a rule from scratch.

A typical custom rule sheet covers six to ten bills, three to five edge-case rules, and a settlement convention (which day of the month, through which method). Anything longer than one page stops being read. Anything shorter usually means a bill has been forgotten.

When custom is the right call

The custom approach earns its keep when at least one of the following is true.

• Long tenancies. Twelve months or more. Over that horizon, even small monthly unfairnesses compound, and the time spent designing the rule sheet pays back many times over.
• Mixed living situations. One flatmate works from home, one travels constantly, one is on an exchange program. No single method handles all three; custom lets each utility match its right method.
• Heating-heavy flats with very different room sizes. When winter gas dominates the budget and one bedroom is twice the size of another, heating clearly needs per-square-meter, but water clearly does not. Custom is how you say both.
• Groups where one or more disputes already happened. If the first month’s bill produced an argument, a written rule sheet is the cheapest way to prevent the next ten.
• High-stakes setups. Joint leases with shared deposits, flats with expensive utilities, situations where the flatmates barely knew each other before moving in. The more there is to lose, the more the written rule pays off.

When custom is overkill

The same logic flips when the flat is simple enough that a single rule covers everything fairly.

• Short-term sublets. Two or three months. The setup cost of a custom sheet is not recoverable over that horizon — equal split or per-day, picked in five minutes, is fine.
• Already-equal usage patterns. Two flatmates with similar rooms, similar hours, similar habits. Equal split is exactly right and a custom sheet just adds paperwork.
• Groups that trust each other strongly. Friends or family members who have lived together before, where the bill conversation is genuinely casual. Custom can feel forced where trust is already established — though even here, a one-page sheet often saves grief later.
• Single dominant utility. A flat with rent and a flat-rate internet bill and nothing else. There is nothing to design rules around.

A useful test: list every recurring bill on a piece of paper, ask whether the same split method makes sense for all of them, and if the answer is yes, you do not need custom. If the answer is no, you do.

How to design the rule sheet — a five-step process

The design works as a sequence. Skipping a step usually shows up as an argument three months later.

Step 1 — Inventory all expenses

Before deciding any methods, list every recurring and periodic cost the flat actually pays.

• Recurring bills. Electricity, gas, water, internet, condo or building dues, trash collection where it is billed separately.
• Periodic costs. Cleaning supplies, light bulbs, salt or filter refills, anything bought monthly or quarterly.
• One-time costs. Small repairs, replacement items, occasional shared purchases.

The single biggest cause of bill arguments is a utility nobody mentioned at handover that suddenly shows up. The inventory step prevents that. Spend twenty minutes listing everything together, including the rough monthly amount and the billing cycle of each item.

Step 2 — Choose a method per bill

For each bill on the inventory, pick the split method that matches what the bill actually tracks. The defensible choices for common utilities:

Bill	Default method	Why
Heating	Per square meter	Bigger rooms need more heat
Electricity	Per square meter or equal	Mostly per-room, some personal-device weighting if needed
Water	Equal or per-day	Scales with people, not space
Internet	Equal	Bandwidth does not scale with anything
Condo dues	Equal	Fixed fee per flat
Trash and cleaning	Equal	Per-person consumption
Streaming subscriptions	Equal or excluded	Personal accounts handled separately

Build a table like this in your rule sheet. The rationale column on the right matters: when a new flatmate joins six months in, the rationale is what teaches them the logic without having to relitigate every decision.

Step 3 — Address edge cases upfront

Edge cases happen in every flat. The rules for handling them are easier to agree on at handover, when nobody is invested in any particular outcome, than later when an actual situation makes one flatmate the winner and another the loser.

The edge cases worth deciding in advance:

• Long absences. What if someone is away more than two weeks? A common rule: their share drops to 50% for utilities that scale with consumption (water, electricity, gas) and stays full for fixed bills (internet, condo dues). The saved amount gets redistributed to those present.
• Mid-month moves. What if a flatmate moves out on the 15th? Apply per-day for that month, regardless of the usual method, so the math is clean.
• Long guest stays. What about a partner or friend staying more than a week? A common rule: counts as 0.5 of a person for utility-share purposes during that week. The host’s effective person-count goes up correspondingly.
• Cleaning supplies and small shared purchases. Who buys them, and how is reimbursement handled? Rotating monthly purchase with no reimbursement is the simplest approach. Shared kitty is the most accurate.
• Bill ownership. Who actually collects the bills, calculates the splits, and chases payment? Either one person takes it on (and the others trust the math, with the bill always visible), or the role rotates monthly.

Three to five edge-case rules are usually enough. More than that and the sheet stops being readable.

Step 4 — Document and sign

The rule sheet only works if it is treated as a real document, not a casual note. The minimum standard:

• One page maximum. If it spills onto a second page, the rules are too detailed or the inventory has too many items.
• Each rule on its own line. No paragraphs of prose. Bullets, table rows, or numbered list.
• No ambiguity. “Heating per square meter” is fine. “Heating roughly per square meter, but we can negotiate” is not.
• Names, date, and signatures at the top. Every flatmate initials the document. A signed PDF, a photographed paper copy, or a confirmed chat message all work as evidence.
• One copy per flatmate plus a shared digital copy. Stored where everyone can find it without asking. Shared documents folder, pinned chat message, or a dedicated app.
• Sign at handover, not later. Once a dispute has started, getting everyone to agree on rules becomes orders of magnitude harder. The same rules that take fifteen minutes on move-in day take hours after the first argument.

Step 5 — Review and update

A rule sheet is not a contract carved in stone. Conditions change — a flatmate moves out, someone starts working from home, the heating bill turns out larger than anyone expected. The sheet should have a built-in review rhythm.

• Revisit every six months. Even if nothing has changed, a quick review confirms the rules still match the reality. Most reviews end in “no change,” which is fine.
• Trigger a review whenever a flatmate changes. New flatmate means new lifestyle, new room allocation, possibly a new split entirely. Reset the rule sheet at the moment of change, not three months later.
• Document every update in writing, through the same signing process. Verbal updates do not survive memory drift.
• Past rules apply to past months only. If the rule changes in October, the old rule still applies to September bills that arrive in late October. Never recalculate past bills under new rules — that is how trust breaks.

A sample rule sheet

Here is what a working custom rule sheet looks like for a three-flatmate flat of 80 square meters total. Names anonymized, all amounts illustrative.

Flatmates: A, B, C
Bedrooms: 18, 15, 12 m² respectively
Date signed: 1 September 2026

1. Electricity: equal split (three ways) — flat is small,
   usage similar across flatmates
2. Heating gas: per square meter of bedroom (40% / 33% / 27%)
3. Water: equal split — usage individual but bill small
4. Internet (50 per month): equal split — flat rate
5. Condo dues (40 per month): equal split — fixed fee
6. Cleaning supplies: rotated monthly purchase, no reimbursement
7. Guest stays longer than 7 nights in a month:
   counted as 0.5 person for water and electricity that month
8. Absences longer than 14 consecutive days: 50% of utility share,
   savings redistributed to those present
9. Monthly settlement: 5th of each month via shared app
10. Late payment: 5-day grace period, then a small agreed surcharge
    on the overdue amount

Review: every 6 months OR when a flatmate changes
Signatures: A_____ B_____ C_____

That is the entire document. Ten rules, one page, signed at handover. It covers the inventory, the methods, the edge cases, the settlement convention, and the review trigger — everything the flat needs to handle bills mechanically for the next twelve months.

The specific numbers above are illustrative; what holds across flats is the structure. Three rules for the recurring bills with their methods, three to five for the edge cases, two for the operational side (settlement and late payment), and one line specifying the review trigger.

If you want this document to live somewhere other than a shared notes file, Plinthos handles custom splits as one of its four built-in methods. You define the rule per utility, upload each bill (or use OCR to extract the amount), and the app calculates each flatmate’s share from the rules you set — every flatmate sees the original document next to their own portion. See how it works → /en/#how-it-works.

How to handle pushback

Some flatmates will resist the formality. The common objections and how to address them:

• “We are friends, this is awkward.” Frame the rule sheet as protection, not distrust. If anyone moves out unexpectedly, or if the heating bill arrives larger than expected, the signed sheet saves everyone from calculating from scratch under pressure.
• “This implies we do not trust each other.” It implies the opposite — that everyone trusts the same set of rules to apply, instead of relying on whoever does the math being neutral every single month.
• “We do not need this much detail.” The detail is what makes the document usable later. Vague rules (“we will figure it out”) produce arguments; specific rules (“heating per square meter, water equal, internet equal”) do not.
• “It feels like a contract.” It is an internal flatmate agreement, not the lease itself. In most jurisdictions it does not affect the obligations toward the landlord, and it can be updated by mutual consent at any time. The signature is evidence of the agreement among flatmates, not a tenancy document.

The shortest version of the argument: ambiguity creates more conflict than written rules. A one-page document signed at move-in saves dozens of arguments over a year.

Common mistakes

These come up across most flats that try the custom approach for the first time.

• Letting the rule stay verbal. Memory drift kicks in around the third month. What everyone “remembers agreeing to” diverges, and there is no document to settle it.
• Adding rules retroactively. Inventing a new rule mid-tenancy to handle a situation that has already happened is the fastest way to create resentment. New rules apply going forward only.
• Making the sheet too complex. Two pages or more means nobody reads it. The rules become invisible, which is the same as not having them.
• Method without rationale. Listing the split methods without explaining the “why” makes edge cases impossible to handle. Each rule needs a short reason.
• One person decides for the flat. The rule sheet has to be co-designed. A document handed to flatmates for signature, with no input from them, is not a rule sheet — it is a unilateral instruction, and it does not survive the first disagreement.
• Skipping the inventory. Picking methods before listing every bill produces a sheet with gaps. The bills not on the sheet become arguments.
• No settlement convention. Without a specific day of the month for paying up, payments drift and “who owes what” becomes a recurring chore. Pick a date, stick to it.

A related read worth keeping handy when the design conversation gets tense: Flatmate conflict mediation covers how to handle the discussions that come up around money specifically. The underlying philosophy — fair beats equal — is what justifies the per-utility design in the first place, and the small costs guide handles the items too small to land in a rule sheet but too persistent to ignore.

Frequently asked questions

How long should the rule sheet actually be?

One page. Half a page if the flat is simple. Anything longer becomes unread, which means the rules effectively do not exist. If you find yourself adding rules to handle every conceivable scenario, you are designing for the wrong problem — the goal is to cover 90% of situations clearly and leave the rest to good-faith conversation.

Can we mix per-day and per-square-meter on the same bill?

Yes, and some flats do exactly that for heating. Per-square-meter for the baseline (because bigger rooms need more heat) plus a per-day adjustment for the people present (because empty rooms do not need full warmth). It is more complex to compute but defensible. Just write the formula clearly and stick with it.

Does the rule sheet replace anything in the lease?

No. The rule sheet is an internal flatmate agreement and has no effect on the lease. Obligations to the landlord — rent, deposit, utility accounts in someone’s name — still flow through whatever the lease says. The rule sheet only governs how the flatmates split shared costs among themselves. In most jurisdictions, internal flatmate agreements are private arrangements, not enforceable tenancy documents.

What if a flatmate refuses to sign?

That is a signal worth taking seriously. Either the flatmate has a substantive objection to a specific rule (worth discussing and possibly amending), or they object to the existence of a written document at all (which usually means custom is not the right method for this flat). In the second case, fall back to a simpler method — equal or per-day — and accept that disputes will be handled case-by-case instead of by rule.

How do we handle a bill that was not on the original sheet?

Two options. Pick the closest-matching rule from the existing sheet and apply it (“this new utility is like internet, so equal split”), or call a quick review to add it formally. Either is fine; what is not fine is letting the new bill become an unresolved dispute. Decide quickly, document the decision, move on.

What is the lightest possible version of this?

For a stable flat of two or three flatmates with similar lifestyles, the minimum viable rule sheet is four lines: split method for utilities that scale with consumption, split method for fixed fees, settlement date, late-payment convention. Signed at move-in, stored in the shared chat. Total time to design: ten minutes. Total arguments prevented: most of them.

Closing

Custom rule sheets are the fairest of the four splitting methods because they let every bill find its right method, but the fairness only materializes if the sheet is actually written, actually signed, and actually applied. The work is upfront: inventory the bills, pick the methods, address the edge cases, sign at handover, review every six months. After that, the math is mechanical and the conversations about money mostly stop happening.

The right rule sheet for any specific flat is the one the flatmates can read in two minutes, defend in five, and apply for twelve months without needing to redesign. Pick deliberately, write it down on a single page, and most of the recurring bill conversation disappears.

If you want the rule sheet to live somewhere it can actually be applied automatically, Plinthos handles custom splits alongside the other three built-in methods, keeps the original bill visible next to each flatmate’s share, and tracks payments month by month. See features →

This article describes general practices for designing a flatmate rule sheet for splitting utility bills. Internal flatmate agreements are not legal contracts and do not replace lease obligations toward the landlord. For lease-level questions, in most jurisdictions you will need to check local rental rules or consult a tenant association.