VAT and Contractor Arbitrage: What UK Operators Get Wrong

VAT in a UK contractor arbitrage business comes down to two things: when you must register, and how the domestic reverse charge changes the way sub invoices work. Get those right and you are well ahead of most operators. Get them wrong and you face backdated VAT bills, penalties, or an incorrect paper trail.

(All rules below are UK-specific. The construction arbitrage model works in any country - the compliance details do not.)

When does VAT registration apply to a contractor arbitrage business?#

You must register for VAT once your taxable turnover exceeds £90,000 in any rolling 12-month period. The trap is in those last four words: rolling 12-month period.

HMRC does not measure turnover by calendar year. It looks at any rolling window - so the 12 months ending on the last day of every month. If your billings for the most recent 12 months tip over £90,000 at any point during the year, you have 30 days to notify HMRC and then you must be registered. Leaving it until your April year-end means you may already be months late.

For a contractor arbitrage operator, the number that counts is your gross billings - what you charge the client, not your profit on the job. A £15,000 bathroom renovation that you subcontracted for £10,500 adds £15,000 to your turnover, not £4,500. Six jobs like that and you are approaching the threshold fast.

Register for VAT with HMRC before you hit the threshold if you can see it coming. Voluntary registration is allowed at any turnover level, and being registered early gives you the ability to reclaim VAT on your business costs - software, insurance, tools, vehicle expenses - before you are forced to do it.

What is the domestic reverse charge and why does it change how sub invoices work?#

The VAT domestic reverse charge for building and construction services has applied in the UK since March 2021. It was introduced to close a specific fraud where contractors collected VAT from clients and disappeared without paying it to HMRC.

Here is what it means in practice for a registered contractor arbitrage operator:

When you pay a VAT-registered subcontractor for construction work that falls within CIS scope, they do not charge you VAT on their invoice. Instead, you account for the VAT yourself - as both an output and an input on your VAT return. The two entries cancel each other out, so there is no net cost to you, but the accounting entry must be there.

The reverse charge applies only within the B2B supply chain. When you raise an invoice to your end client - the homeowner, the landlord, the property company - you charge VAT in the normal way. The end client pays you including VAT, and you remit that VAT to HMRC.

So the chain looks like this:

• Sub invoices you: no VAT (reverse charge applies - you account for it)
• You invoice client: normal VAT at 20%

The mistake most operators make is either assuming all sub invoices work normally (and not accounting for the reverse charge) or assuming the reverse charge applies to everything including client invoices (and forgetting to charge the client). Both are wrong.

The five VAT mistakes UK contractor arbitrage operators make#

1. Monitoring turnover too loosely. Using a rough sense of what jobs came in this year, rather than tracking the rolling 12 months. By the time you realise you crossed £90,000, you may owe backdated VAT on several months of invoices you raised without it. The fix is simple: check your running 12-month total every month.

2. Not asking whether a sub is VAT-registered before they invoice. The reverse charge only applies when both parties are VAT-registered. If your sub is below the registration threshold and not registered, they charge you normal VAT (or nothing, if they are unregistered). Assuming all subs operate the same way causes errors on both sides. Ask every new sub upfront.

3. Misunderstanding the end user exception. The reverse charge does not apply to end users - businesses that receive construction services but do not make onward supplies of construction work. A large supermarket chain having their roof fixed is an end user. A housing association with no in-house build function may be an end user. If you are working for an end user who has notified you in writing of that status, the reverse charge does not apply to that supply and normal VAT rules kick in. Getting this classification wrong costs money.

4. Assuming the VAT Flat Rate Scheme helps. It almost certainly does not for contractor arbitrage operators. HMRC classifies businesses as "limited cost traders" when their spend on goods is less than 2% of turnover or less than £1,000 per year. For a contractor arbitrage operator whose costs are primarily subcontractor labour - not goods - the flat rate climbs to 16.5%, which is worse than standard VAT accounting for almost every operator in this model. Do not assume the Flat Rate Scheme is a shortcut here.

5. Treating VAT as your money before the return. VAT on client invoices is not your income - it is HMRC's money sitting temporarily in your account. The single most common cash flow crisis in early contractor arbitrage businesses is spending the VAT before the quarterly return comes in. Keep it in a separate account from the moment the payment lands.

The cash flow effect of VAT registration#

There is a genuine upside to being VAT-registered that most operators underestimate. Once registered, you can reclaim the VAT on your own business costs: your liability insurance, your CRM software, your accounting fees, your vehicle business use, your laptop. These add up.

The bigger adjustment is inwards cash flow. Before VAT registration, you priced jobs without VAT. After registration, your quotes to VAT-registered clients go up by 20% - but they can reclaim it, so it is neutral to them. Your quotes to non-VAT-registered clients (many private homeowners) go up by 20% and they feel it. You either absorb some of that increase by trimming margin, or you pass it on and accept losing some price-sensitive clients.

Running the numbers before you hit the threshold is worth it. At £80,000-£85,000 in turnover, get your accountant to model what registration looks like on your client mix.

For the broader picture of how margins and money work in this model, see how much money contractor arbitrage actually makes and how construction arbitrage taxes work.

Setting the business up to handle VAT cleanly#

The setup required is not heavy, but it must be in place from registration day:

• Use accounting software that handles the reverse charge - most UK construction accountants use Xero or Sage, both of which can handle reverse charge entries correctly.
• Get a UK accountant who works with construction businesses, not a generalist. The domestic reverse charge and CIS interact in ways that a general small-business accountant does not always understand.
• Register for CIS as a contractor with HMRC before paying your first sub - the reverse charge only applies to CIS-reportable work, so your CIS registration and VAT registration are linked in practice.

The is contractor arbitrage legal in the UK post covers the full CIS registration requirements alongside VAT. The sole trader or limited company guide covers how your business structure affects the VAT picture.

VAT is manageable - but it requires the right systems#

The operators who get into trouble with VAT are almost never the ones who tried to evade it. They are the ones who got busy, tracked turnover loosely, and discovered they had been operating over the threshold without registering. Or the ones who paid a sub incorrectly for twelve months and then had to rework a year of VAT returns.

The model itself is VAT-neutral once you are registered and have the reverse charge working correctly. HMRC is not trying to catch contractor arbitrage operators - it introduced the reverse charge to catch fraud higher up the chain. Comply properly and VAT becomes an administrative task, not a risk.

If you want to stay current as VAT thresholds and reverse charge rules change, join the mailing list - new posts and rule updates land there first.

Last checked: 14 September 2026.