You quote a day rate, win the contract, and then the questions start. How much of that rate actually lands in your pocket? Which costs can you put through the books? And the big one for any engineering contractor: are you inside or outside IR35?
This guide is for consulting engineers working through a limited company or as a sole trader, whether you're a structural, civil, mechanical, electrical or software engineer. We'll walk through how your day rate is taxed, the expenses you can legitimately claim, and how IR35 decides who pays the tax on each contract.
All figures are for the 2025/26 tax year and come straight from HMRC. We'll show you the maths, not just the headline.
How is a consulting engineer taxed?
It depends on how you work, and the structure changes the tax entirely.
If you operate as a sole trader, you pay Income Tax and self-employed National Insurance on your profit (income minus allowable expenses). For 2025/26, Class 4 NIC is 6% on profits between the Lower Profits Limit of £12,570 and the Upper Profits Limit of £50,270, then 2% above that.
If you run a limited company, the company pays Corporation Tax on its profit. You then take money out as a mix of salary and dividends, each taxed differently in your own hands. The company itself is the contracting party, which is exactly why IR35 exists (more on that below).
Most engineering contractors on day rates use a limited company, because agencies and end clients usually want to contract with a company rather than an individual. So the rest of this guide leans towards the limited-company route, while flagging the sole-trader position where it differs.
If you want the full structural comparison, our guide for consulting and engineering professionals breaks down when each setup makes sense.
How much of my day rate do I actually keep?
This is the question that matters most when you're negotiating. A day rate is gross. Tax, NIC and your costs all come out before you see the rest.
The honest answer is that take-home depends on your structure, your other income, how much you leave in the company, and whether the contract is inside or outside IR35. Rather than hand-wave, here's a worked example.
Illustrative example: an outside-IR35 limited company contractor
Sam is a mechanical engineer contracting through her own limited company. The contract is genuinely outside IR35. She bills £450 a day and works 220 billable days in the year, so her company turns over £99,000. She has £9,000 of allowable business costs (insurance, software, accountancy, equipment, travel to varied sites). She takes a small salary of £12,570 and the rest as dividends.
Here's the company-level position for 2025/26.
| Item | Amount |
|---|---|
| Turnover (220 days × £450) | £99,000 |
| Less allowable expenses | £9,000 |
| Less director's salary | £12,570 |
| Profit before Corporation Tax | £77,430 |
| Corporation Tax (see note) | £16,769 |
| Profit available as dividends | £60,661 |
A note on the Corporation Tax figure. For Financial Year 2025, profits up to £50,000 are taxed at the 19% small profits rate and profits over £250,000 at the 25% main rate, with Marginal Relief easing the gap in between. Sam's £77,430 profit sits in the marginal band, so her effective rate lands between 19% and 25%. The £16,769 above is the main rate (25% of £77,430 = £19,358) less Marginal Relief, using the standard 3/200 fraction on the gap between her profit and the £250,000 upper limit (3/200 of £172,570 = £2,589). The exact figure will depend on her precise profit, which is why we always run this through proper software at year end.
Now the personal side. Sam draws a £12,570 salary plus £60,661 in dividends, giving total income of £73,231. The salary uses up her Personal Allowance, so the dividends are taxed as follows for 2025/26.
| Dividend band | Amount | Rate | Tax |
|---|---|---|---|
| Dividend allowance | £500 | 0% | £0 |
| Remaining basic-rate room | £37,200 | 8.75% | £3,255 |
| Higher-rate dividends | £22,961 | 33.75% | £7,749 |
| Total dividend tax | £11,004 |
How the basic-rate room is worked out: the basic-rate band covers taxable income up to £37,700. The first £500 of dividends is covered by the dividend allowance but still uses up band, so £37,200 of dividends is taxed at the 8.75% ordinary rate. The remaining £22,961 falls into the higher-rate band and is taxed at 33.75%.
So from a £99,000 turnover, Sam's company pays roughly £16,769 in Corporation Tax and she personally pays about £11,004 in dividend tax. That leaves her with around £62,227 in the bank (salary of £12,570 plus dividends of £60,661 minus £11,004 dividend tax), before counting the £9,000 of costs the business genuinely incurred on her behalf.
This is illustrative only. Your salary level, pension contributions and other income all move the numbers. To model your own rate quickly, try our take-home pay calculator and then talk to us about the bits a calculator can't see.
Why inside IR35 changes everything
If that same contract were caught by IR35, the picture is very different. The fee paid to Sam's company would be treated as employment income. Income Tax and employee National Insurance would be deducted, plus the engager pays employer NIC on top. The dividend route effectively closes, and her take-home drops noticeably. That's why the IR35 question is worth real attention, not a shrug.
What expenses can a consulting engineer claim?
The rule is simple to state and easy to get wrong. A cost is allowable if it's incurred wholly and exclusively for the purposes of your business. If something is part personal and part business, you only claim the business proportion.
Here are the costs that come up most often for engineering contractors, with how they're usually treated.
| Cost | Usually allowable? | Notes |
|---|---|---|
| Professional indemnity insurance | Yes | Treated as a financial/business cost by HMRC |
| Accountancy and bookkeeping fees | Yes | A clear business cost |
| Engineering software and licences | Yes | CAD, analysis and design tools used for work |
| Equipment (laptop, instruments, tools) | Yes | Via capital allowances or full expensing for companies |
| Professional body subscriptions | Usually | Where the body is on HMRC's approved list (for example relevant chartered institutions) |
| Training and refresher courses | Often | Refresher and updating courses tied to your existing work; not the cost of a brand-new qualification |
| Travel to a temporary workplace | Yes | Fuel, parking, train or bus fares for genuine business travel |
| Use of home as office | Yes (proportion) | A reasonable share of heating, electricity, internet and phone |
| Commuting to a permanent workplace | No | Ordinary commuting isn't allowable |
| Anything taken for personal use | No | Personal drawings are never an expense |
A couple of points engineers get caught on. First, the 24-month rule on travel: once you expect to be at one site for more than two years, it stops being a temporary workplace and travel there is no longer claimable. Second, training. Keeping your skills current is fine, but HMRC generally won't allow the cost of training that gives you a new qualification or a new trade.
Equipment is a category in its own right. Companies can often write off qualifying plant and machinery in full in the year of purchase under full expensing or the Annual Investment Allowance, which is set at £1,000,000. That can include instruments, laptops and other kit you buy for the work.
Getting expenses right is the difference between a clean tax bill and an awkward HMRC enquiry. Our tax advisory service helps engineering contractors set up a sensible, defensible expenses policy from day one.
What is IR35 and why does it matter for engineers?
IR35, also called the off-payroll working rules, exists to stop people working like employees but paying tax like a business by routing their fee through a limited company. If the rules apply, HMRC treats the engagement as employment for tax, and far more tax becomes due.
Engineering is squarely in HMRC's sights here, because so many engineers contract through a personal company on long site-based or office-based assignments that can look a lot like employment.
Three tests, drawn from decades of case law, sit at the heart of any IR35 assessment:
- Personal service and substitution. Must you personally do the work, or could you genuinely send a suitably qualified substitute in your place? A real right of substitution points towards self-employment.
- Control. Does the client decide what you do, and how, when and where you do it? Heavy day-to-day control points towards employment.
- Mutuality of obligation. Is the client obliged to offer you work and are you obliged to accept it, beyond the specific task agreed? Ongoing mutual obligation looks like employment.
No single test is decisive. HMRC and the courts stand back and look at the overall picture, including financial risk, who provides the equipment, and whether you're genuinely "part and parcel" of the client's organisation.
Who decides your IR35 status?
This is where the size of your client matters.
If you contract for a medium or large private-sector client, the client decides your status and must give you a Status Determination Statement explaining their reasoning. If the client decides you're inside, the party paying your company (often an agency) deducts tax and NIC before paying you.
If you contract for a small private-sector client, the responsibility stays with you and your own company. You assess each contract and account for the tax accordingly. A private-sector client is currently treated as medium or large if it meets two or more of these: turnover over £10.2 million, balance sheet total over £5.1 million, or more than 50 employees. Companies below that are small.
One thing to watch: the Companies Act size thresholds were increased by regulations for financial years beginning on or after 6 April 2025 (turnover up to £15 million, balance sheet up to £7.5 million, employees up to 50). These feed into the off-payroll size test on a delayed basis, so check the current position for any given tax year rather than assuming. If you're unsure which side of the line a client sits, ask them, they're obliged to confirm their size on request.
For a deeper walk-through of the rules and how agencies fit in, see our guides for IR35 contractors and for contractors more broadly.
Am I inside or outside IR35? A decision guide
You can't tell from your job title. A "consulting engineer" can be either side of the line depending on the actual working arrangement. Work through these questions for each contract.
- Could you send a substitute? If you have a genuine, unfettered right to send another qualified engineer and the client must accept them, that's a strong pointer to outside.
- Who controls the work? If you decide how and when you deliver, work to your own methods, and aren't slotted into the client's management chain, that points outside. If you're told when to clock in and supervised like staff, that points inside.
- Is there mutuality of obligation? If the client has no duty to give you more work after the task, and you have no duty to take it, that points outside.
- Do you carry business risk? Fixing defects in your own time, providing your own kit and indemnity insurance, and risking your own money all point outside.
- Are you part of the furniture? Staff perks, a permanent desk, a company email signature and line-management all point inside.
If most of your honest answers point one way, that's your likely status, but borderline cases need proper review. HMRC's free Check Employment Status for Tax (CEST) tool is a starting point, and your written contract should match what actually happens on the ground. A contract that says one thing while reality says another won't protect you.
Talk to a specialist before you sign. Getting IR35 wrong on an engineering contract can cost you thousands in back tax and penalties. Book a free 20-minute call with a Zmartly accountant and we'll review your contract and working practices before your next assignment. Get in touch with Zmartly.
If you're scaling up, bringing in subcontractors, or working with US clients, our outsourced US CFO services can support cross-border engineering consultancies too.
Frequently asked questions
Do consulting engineers need a limited company?
No, but most day-rate contractors use one. A limited company is often required by agencies and end clients, and it can be more tax-efficient than sole-trader status once profits are reasonable. The right answer depends on your income, your other commitments and whether your contracts tend to fall inside or outside IR35.
Is professional indemnity insurance tax-deductible for engineers?
Yes. HMRC treats professional indemnity insurance as an allowable financial cost where it's taken out for your business, so it reduces your taxable profit. Most engaging clients require it anyway, so it's both necessary and deductible.
Can I claim travel to a client's site?
You can claim travel to a temporary workplace, including fuel, parking and rail fares. But once you expect to be based at one site for more than 24 months, it counts as a permanent workplace and the travel stops being allowable. Ordinary commuting to a permanent base is never claimable.
What happens if my engineering contract is inside IR35?
The fee your company receives is treated as employment income. Income Tax and employee National Insurance are deducted before you're paid, and the engager pays employer NIC on top. You can't use the salary-and-dividend mix to the same effect, so your take-home is lower than on an equivalent outside-IR35 contract.
Does IR35 apply if my client is a small company?
The off-payroll rules still exist, but for a small private-sector client the responsibility for assessing status and accounting for any tax shifts to you and your own company, rather than the client. You should assess each contract on its facts. If you're not sure whether a client counts as small, you can ask them to confirm their size.
