The CT600 is the Company Tax Return form filed with HMRC. Tax Optimiser completes it for you from the figures you enter in the Corp Tax workspace — the CT600 Boxes section of the calc screen shows exactly which boxes your figures populate, and each box there links to its entry below. Box numbering follows the CT600 (2026) form. For HMRC's own box-by-box notes see the Company Tax Return guide, on which this reference draws (Open Government Licence v3.0).
Company information
Box 1 — Company name
The registered company name. Tax Optimiser fills this from the accounting period's registered company name.
Box 2 — Company registration number
The Companies House registration number, from the organisation's details.
Box 3 — Tax reference
The company's 10-digit Unique Taxpayer Reference (UTR) from its HMRC record.
Box 4 — Type of company
A numeric code describing the kind of company (for example members' club, charity, property management). Set the company type on the Send Tax Return step of the submission wizard; most trading companies leave it as the default.
Boxes 5 to 8 — Northern Ireland
Boxes 5 (NI trading activity), 6 (SME), 7 (NI employer) and 8 (special circumstances) only apply to companies within the Northern Ireland Corporation Tax regime. Tax Optimiser does not currently set them.
About this return
Boxes 30 and 35 — Period covered by this return
The start and end dates of the return period. A return can cover at most 12 months, so an accounting period longer than 12 months is split into two returns — Tax Optimiser does this automatically and shows Period 1 and Period 2 side by side.
Box 40 — A repayment is due for this return period
Ticked when the return shows tax overpaid (box 605), for example after a loss carry-back or payable credit claim.
Box 45 — Claim or relief affecting an earlier period
Ticked when this return makes a claim that changes an earlier period — most commonly carrying a trading loss back to the previous year from the Losses section.
Box 50 — Making more than one return now
Ticked when more than one return for the company is being delivered at the same time, as happens with a long period of account split into two returns.
Box 55 — This return contains estimated figures
Ticked when any figures are estimates to be revised later. Tax Optimiser sets this from the “contains estimates” flag on the General section.
Box 60 — Company part of a group that is not small
Ticked when the company is a member of a group that is not small — relevant to transfer pricing and senior accounting officer rules.
Box 65 — Notice of disclosable avoidance schemes
Ticked when the company must report a Disclosure of Tax Avoidance Schemes (DOTAS) reference — entered in the Tax Avoidance section, which also drives the CT600J supplementary page.
Box 70 — Compensating adjustment claimed
Transfer pricing: ticked when the company claims a compensating adjustment for a counterparty's transfer-pricing adjustment.
Box 75 — Company qualifies for SME exemption
Transfer pricing: ticked when the company relies on the small and medium-sized enterprise exemption from the transfer-pricing rules.
Boxes 80 to 90 — Accounts and computations
Box 80 confirms accounts and computations for this period are attached — Tax Optimiser always attaches your statutory accounts and its own computation, so box 80 is ticked on every return. Box 85 is used instead when the accounts cover a different period (for example the second return of a long period of account), and box 90 explains any accounts not attached.
Supplementary pages enclosed
Box 95 — CT600A, loans to participators
Ticked when the return includes the CT600A for loans by a close company to participators (directors/shareholders and their associates). Filled automatically when you enter loans in the Loan to Participators section.
Box 96 — CT600P, creative industries
Ticked when the return includes the CT600P for creative industry claims — driven by entries in the Creative Industries section.
Boxes 100 to 144 — other supplementary pages
One tick per supplementary page enclosed: CT600B controlled foreign companies (box 100), CT600C group and consortium (105), CT600D insurance (110), CT600E charities and CASCs (115 — set by the Charity/CASC company type), CT600F tonnage tax (120), CT600G Northern Ireland (125), CT600H cross-border royalties (130), CT600I ring fence trades (135), CT600J disclosed avoidance schemes (140), CT600K restitution tax (141), CT600L Research and Development (142), CT600M freeports and investment zones (143) and CT600N Residential Property Developer Tax (144).
Turnover
Box 145 — Total turnover from trade
The turnover of the trade for the return period, from your Profit & Loss figures (apportioned by days when the period is split into two returns).
Box 150 — No recognised turnover
Ticked only by banks, building societies, insurance companies and other financial concerns that have no recognised turnover and so leave box 145 blank.
Income
Box 155 — Trading profits
The adjusted trading profit for tax: accounting profit plus disallowable expenses, less capital allowances and other adjustments. Tax Optimiser derives it from the P&L, Assets and adjustment sections — see the computation document for the full working.
Box 160 — Trading losses brought forward set against trading profits
Pre-April-2017 trading losses brought forward, which can only be used against profits of the same trade. Entered in the Losses section.
Box 165 — Net trading profits
Box 155 minus box 160. Calculated automatically.
Box 170 — Interest and profits from non-trading loan relationships
Bank and other interest receivable and other non-trading loan relationship credits, net of non-trading deficits — from the Non Trade Credit and Non Trade Debit sections.
Box 172 — Deficit carried back included in box 170
Ticked when box 170 is stated net of a non-trading deficit carried back from a later period.
Box 175 — Annual payments not otherwise charged
Annual payments received that are taxable but not caught by any other heading and from which Income Tax has not been deducted.
Box 180 — Non-exempt dividends from non-UK resident companies
Overseas dividends and distributions that do not qualify for the (usual) dividend exemption.
Box 185 — Income from which Income Tax has been deducted
The gross amount of income received under deduction of Income Tax; the tax deducted itself is credited at box 515. Entered in the Income Tax Suffered section.
Box 190 — Income from a property business
Profits of a UK property business for the period, from the Non Trade Income section.
Box 195 — Non-trading gains on intangible fixed assets
Credits on intangible fixed assets (goodwill, IP) held for non-trading purposes.
Box 200 — Tonnage tax profits
Profits of a shipping company that has elected into the tonnage tax regime (with the CT600F).
Box 205 — Income not falling under any other heading
Miscellaneous taxable income that fits nowhere else on the form — entered in the Other Income section.
Chargeable gains
Box 210 — Gross chargeable gains
Chargeable gains for the period before losses.
Box 215 — Allowable losses
Capital losses of the period and losses brought forward, up to the amount of gains in box 210.
Box 220 — Net chargeable gains
Box 210 minus box 215. Calculated automatically.
Profits before other deductions and reliefs
Box 225 — Losses brought forward against certain investment income
A narrow relief for losses brought forward set against certain investment income (mainly relevant to companies with investment business).
Box 230 — Non-trade deficits brought forward
Non-trading loan relationship deficits brought forward and set against non-trading profits of this period.
Box 235 — Profits before other deductions and reliefs
The net sum of boxes 165 to 205 and 220, minus boxes 225 and 230. Calculated automatically.
Deductions and reliefs
Box 240 — Losses on unquoted shares
Share loss relief on qualifying unquoted shares.
Box 245 — Management expenses
Expenses of managing a company with investment business.
Box 250 — UK property business losses
UK property business losses of this or a previous period set against total profits.
Box 255 — Capital allowances for management of the business
Capital allowances on assets used in managing the company's investment business.
Box 260 — Non-trade deficits for this period
This period's non-trading loan relationship deficit claimed against total profits — from the Non Trade Debit section.
Box 263 — Carried forward non-trade deficits
Post-April-2017 non-trading deficits brought forward and claimed against total profits.
Box 265 — Non-trading losses on intangible fixed assets
Non-trading intangibles losses set against total profits.
Box 275 — Trading losses of this or a later period
Trading losses of this period (sideways relief) or of a later period (carry-back) set against total profits. Tax Optimiser fills this from the loss claims you make in the Losses section.
Box 280 — Amounts carried back from later periods
Ticked when box 275 includes a loss carried back from a later accounting period into this one.
Box 285 — Trading losses carried forward claimed against total profits
Post-April-2017 trading losses brought forward and claimed against total profits (rather than only trade profits).
Box 290 — Non-trade capital allowances
Capital allowances on non-trading assets, set against total profits.
Box 295 — Total of deductions and reliefs
Total of boxes 240 to 275, 285 and 290. Calculated automatically.
Box 300 — Profits before qualifying donations and group relief
Box 235 minus box 295. Calculated automatically.
Box 305 — Qualifying donations
Qualifying charitable donations paid in the period, from the Donations section. Remember not to also disallow donations in the P&L — the relief is given here.
Box 310 — Group relief
Current-period losses surrendered by 75% group companies and claimed against this company's profits, from the Losses section.
Box 312 — Group relief for carried forward losses
Group relief claimed for another group company's post-April-2017 carried-forward losses.
Box 315 — Profits chargeable to Corporation Tax
Box 300 minus boxes 305, 310 and 312 — the figure tax is charged on. Calculated automatically.
Box 320 — Ring fence profits included
Oil and gas ring fence profits included in box 315.
Box 325 — Northern Ireland profits included
Profits within the Northern Ireland Corporation Tax regime included in box 315.
Tax calculation
Boxes 326 to 328 — Associated companies
The number of associated companies in this period (326), or in each financial year (327/328) where the period spans two financial years with different limits. Associated companies share the small-profits and marginal-relief limits; set the count in the General section.
Box 329 — Small profits rate or marginal relief
Ticked when the company is chargeable at the small profits rate or entitled to marginal relief (periods ending after 31 March 2023). Tax Optimiser works this out from the rate bands applied.
Boxes 330 and 380 — Financial year
The financial year (starting 1 April) each row of the tax table relates to. A return period spanning 31 March uses both columns, and Tax Optimiser apportions profits between them by days automatically.
Boxes 335, 350, 365, 385, 400 and 415 — Amount of profit
The slice of chargeable profit taxed in that financial year at the rate alongside it. Calculated automatically from box 315.
Boxes 340, 355, 370, 390, 405 and 420 — Rate of tax
The Corporation Tax rate applied to that slice: the main rate, or the small profits rate where profits are within the lower limit.
Boxes 345, 360, 375, 395, 410 and 425 — Tax
Profit multiplied by rate for each row, in pounds and pence. Calculated automatically.
Box 430 — Corporation Tax
The total of the tax column (boxes 345, 360, 375, 395, 410 and 425). Calculated automatically.
Box 435 — Marginal relief
Marginal relief where profits fall between the lower and upper limits (from 1 April 2023: £50,000 and £250,000, apportioned for short periods and shared between associated companies). See the computation for the FY-by-FY working.
Box 440 — Corporation Tax chargeable
Box 430 minus box 435. Calculated automatically.
Reliefs and deductions in terms of tax
Box 445 — Community Investment Tax Relief
Tax relief for amounts invested in accredited community development finance institutions.
Box 450 — Double Taxation Relief
Credit for foreign tax paid on income also taxed in the UK.
Box 455 — Underlying rate relief claim
Ticked when box 450 includes relief at the underlying rate on foreign dividends.
Box 460 — Amount carried back from a later period
Ticked when box 450 includes double taxation relief carried back from a later period.
Box 465 — Advance Corporation Tax
Surplus ACT set against the liability under the shadow ACT rules (legacy — pre-1999 distributions).
Box 470 — Total reliefs and deductions in terms of tax
Total of boxes 445, 450 and 465. Calculated automatically.
Coronavirus support schemes and overpayments
Boxes 471 to 474 — CJRS and other coronavirus overpayments
Box 471: CJRS/Job Support Scheme payments received in the period; box 472: the amount the company was actually entitled to; box 473: overpayments already assessed or disclosed; box 474: other coronavirus scheme overpayments. Any net overclaim becomes additional tax at box 526.
Energy levies
Boxes 986 and 987 — Energy (Oil and Gas) Profits Levy and Electricity Generator Levy
Box 986: amounts liable to the Energy (Oil and Gas) Profits Levy; box 987: exceptional generation receipts for the Electricity Generator Levy. Only relevant to oil and gas producers and electricity generators.
Calculation of tax outstanding or overpaid
Box 475 — Net Corporation Tax liability
Box 440 minus box 470. Calculated automatically.
Box 480 — Tax payable on loans to participators
The section 455 charge (33.75%) on loans to participators outstanding at year end, from the Loan to Participators section. Equals box A80 on the CT600A.
Box 485 — CT600A box A70 completed
Ticked when relief for loans repaid, released or written off (section 458) is claimed on the CT600A.
Boxes 490 to 502 — CFC tax, bank levy and other charges
Controlled Foreign Companies tax (490), bank levy (495), bank surcharge (496), Residential Property Developer Tax (497), their total (500), Energy (Oil and Gas) Profits Levy payable (501) and Electricity Generator Levy payable (502). Rare for owner-managed companies.
Box 505 — Supplementary charge (ring fence trades)
The supplementary charge on oil and gas ring fence profits.
Box 510 — Tax chargeable
Total of boxes 475, 480, 500, 501, 502 and 505. Calculated automatically.
Box 515 — Income Tax deducted from gross income
Income Tax suffered at source on income included in profits (see box 185), credited against the Corporation Tax bill. From the Income Tax Suffered section.
Box 520 — Income Tax repayable to the company
Any excess of Income Tax suffered over the tax chargeable, repayable to the company.
Box 525 — Self-assessment of tax payable before restitution tax and coronavirus overpayments
Box 510 minus box 515. Calculated automatically.
Box 526 — Coronavirus overpayment now due
The net coronavirus support scheme overclaim now assessed through the return (boxes 471 + 474 minus 472 and 473).
Box 527 — Restitution tax
The 45% charge on interest awarded on restitution claims against HMRC (with the CT600K).
Box 528 — Self-assessment of tax payable
Total of boxes 525, 526 and 527 — the headline self-assessed liability for the return.
Tax reconciliation
Box 530 — Research and Development credit
R&D tax credit set against the liability, from the R&D Expenditure section (with the CT600L).
Box 540 — Creatives tax credit
Payable creative industry tax credits (film, TV, video games and the cultural reliefs) set against the liability.
Box 541 — Audio-Visual and Video Games expenditure credit
AVEC/VGEC (the expenditure credits replacing the old creative reliefs) set against the liability, from the Creative Industries section.
Box 545 — Total R&D, creatives and AVEC/VGEC credits
Total of boxes 530 to 541. Calculated automatically.
Boxes 550 to 565 — Land remediation, life assurance and first-year tax credits
Land remediation tax credit (550), life assurance company tax credit (555), their total (560) and capital allowances first-year tax credit (565).
Boxes 570 to 580 — Credits payable
The parts of the credits above that exceed the liability and become payable to the company: surplus R&D and creatives credits (570), land remediation or life assurance credit (575) and first-year tax credit (580). Calculated automatically.
Boxes 585 to 590 — Ring fence and Northern Ireland amounts included
Memorandum boxes: ring fence Corporation Tax (585), Northern Ireland Corporation Tax (586) and ring fence supplementary charge (590) included in the liability.
Box 595 — Tax already paid (and not already repaid)
Corporation Tax for this period already paid to HMRC, for example quarterly instalments or early payments.
Box 600 — Tax outstanding
Box 525 minus boxes 545, 560, 565 and 595 — what remains to pay. Calculated automatically.
Box 605 — Tax overpaid
The excess where credits and tax already paid exceed the liability — this drives the repayment claim (boxes 865 onwards) and ticks box 40.
Boxes 610 to 615 — Amounts surrendered to this company
Group tax refunds (610), AVEC/VGEC (614) and R&D expenditure credits (615) surrendered to this company by other group members.
Exporter information
Boxes 616 to 618 — Exports
Whether the company exported goods (616), services (617) or neither (618) outside the UK during the period. Informational only.
Indicators and information
Box 620 — Franked investment income / exempt ABGH distributions
Exempt distributions received from non-group companies — historically used for the marginal relief limits. From the Investment Income section.
Box 625 — Number of 51% group companies
The number of related 51% group companies, which divides the quarterly instalment thresholds. Set in the General section.
Boxes 630 and 631 — Instalment payments
Ticked when the company should have paid by quarterly instalments as a large company (630, profits over £1.5m) or very large company (631, over £20m) — whether or not it actually did. Limits are divided by the box 625 count plus one.
Box 635 — Group payments arrangement
Ticked when the company is within an HMRC group payment arrangement for the period — the flag on the General section.
Box 640 — Intangible assets written down or sold
Ticked when the company wrote down or disposed of intangible fixed assets in the period.
Box 645 — Cross-border royalty payments
Ticked when the company made cross-border royalty payments (see also the CT600H).
Box 647 — Eat Out to Help Out
Reimbursed Eat Out to Help Out discounts included as taxable income (August 2020 scheme).
Enhanced expenditure and tax reliefs
Boxes 650 to 658 — R&D and creatives claim indicators
Who is claiming and what has been filed: SME claim (650), R&D intensive SME (653), large company claim (655), and confirmations that the R&D claim notification form (656), R&D additional information form (657) or creatives additional information form (658) have been submitted to HMRC. The information forms are mandatory — HMRC removes R&D claims made without them.
Box 659 — R&D qualifying expenditure (SME)
The qualifying expenditure behind an SME or R&D-intensive-SME claim, required for periods starting on or after 1 April 2023.
Box 660 — R&D enhanced expenditure
The enhanced (uplifted) R&D expenditure figure from the R&D Expenditure section.
Box 663 — Creatives core expenditure
Core expenditure behind a creative industries claim, from the Creative Industries section.
Box 665 — Creatives additional deduction
The additional deduction given by the creative industry reliefs.
Box 670 — Total enhanced expenditure
Box 660 plus box 665. Calculated automatically.
Boxes 675 and 680 — Subcontracted R&D and vaccine research
R&D enhanced expenditure of an SME on work subcontracted to it by a large company (675) and vaccine research expenditure (680, legacy).
Box 685 — Land remediation enhanced expenditure
The 150% enhanced deduction for qualifying land remediation expenditure.
Capital allowances - in trading profits
Boxes 688 to 730 — Allowances and charges in trading profits
Memorandum boxes analysing the capital allowances and balancing charges already reflected in the trading profit: full expensing (688/689), annual investment allowance (690), super-deduction (691/692), special rate allowance (693/694), special rate pool (695/700), main pool (705/710), structures and buildings (711), electric vehicle charge-points (713/714), business premises renovation (715/720), enterprise zones (721/722), zero-emission goods vehicles (723/724), zero-emission cars (726/727) and other (725/730). Tax Optimiser fills these from the Assets section — AIA, FYA/full expensing, main pool and special rate pool claims.
Capital allowances - not in trading profits
Boxes 733 to 755 — Allowances and charges not in trading profits
The same analysis for allowances on assets outside the trade (for example a property business or managed investments): full expensing (733/734), AIA (735), structures and buildings (736), EV charge-points (737/738), business premises renovation (740/745), super-deduction (741/742), special rate allowance (743/744), enterprise zones (746/747), zero-emission goods vehicles (748/749), zero-emission cars (751/752) and other (750/755).
Qualifying expenditure
Boxes 760 to 775 — Qualifying expenditure
The expenditure incurred in the period on which allowances are claimed: machinery and plant with first-year allowance (760), designated environmentally friendly plant (765), long-life assets and integral features (770), structures and buildings (771), super-deduction plant (772), special rate allowance plant (773) and other machinery and plant (775).
Losses, deficits and excess amounts
Boxes 780 and 785 — Trading losses (UK)
The trading loss arising in this period for trades carried on wholly or partly in the UK (780) and the maximum available to surrender as group relief (785). Tax Optimiser reports the loss arising from your figures; how it is used (carry back, carry forward, sideways) is set in the Losses section.
Box 790 — Trading losses (wholly outside the UK)
Losses of trades carried on wholly outside the UK, which can only be carried forward against the same trade.
Boxes 795 and 800 — Non-trade deficits
The non-trading loan relationship deficit arising (795) and the maximum available for group relief surrender (800).
Boxes 805 and 810 — UK property business losses
The UK property business loss arising (805) and the maximum available for group relief surrender (810).
Boxes 815 to 825 — Other losses
Overseas property business losses (815), losses from miscellaneous transactions (820) and capital losses (825) arising in the period.
Boxes 830 and 835 — Non-trading losses on intangibles
The non-trading intangibles loss arising (830) and the maximum available for group relief surrender (835).
Boxes 840 to 855 — Excess amounts
Amounts unrelievable in this period that could be surrendered as group relief: excess non-trade capital allowances (840), excess qualifying donations (845) and excess management expenses (850 arising / 855 surrenderable).
Northern Ireland information
Boxes 856 to 858 — NI group relief analysis
Only for companies in the Northern Ireland regime: group relief claimed for NI trading losses used against rest-of-UK profits (856), NI losses against NI profits (857) and rest-of-UK losses against NI profits (858).
Overpayments and repayments
Box 860 — Do not repay sums of this amount or less
An optional de-minimis: HMRC will hold rather than repay overpayments at or below the figure entered.
Boxes 865 to 895 — Repayments claimed for this period
How the box 605 overpayment is claimed: repayment of Corporation Tax (865), repayment of Income Tax (870), payable R&D tax credit (875), payable R&D expenditure credit (880), payable creatives credit (885), payable AVEC/VGEC (886), payable land remediation or life assurance credit (890) and payable first-year tax credit (895, closed for periods starting after 31 March 2020). Tax Optimiser derives these from the payable credits on the return, with box 865 as the remaining balance.
Boxes 900 to 915 — Surrender of tax refund within the group
To surrender a refund to another group company: the amount surrendered (900), whether the joint notice is attached (905) or will follow (910), and any amount whose repayment should be held until the notice arrives (915).
Bank details and nominated payee
Boxes 920 to 940 — Bank details
Where any repayment should be sent: bank or building society name (920), branch sort code (925), account number (930), account name (935) and building society reference (940). Entered on the repayment details step before submission.
Box 943 — R&D payable credit condition
Ticked when an R&D payable credit is claimed and one of the conditions listed in HMRC's guide applies to the nominee arrangements.
Boxes 945 to 970 — Payments to a person other than the company
The authority to pay a repayment to a nominee instead of the company: the status of the person giving authority (945), the company name (950), the nominee's name (955) and address (960), the nominee reference (965) and the recipient name (970). Leave blank to have repayments paid to the company.
Declaration
Boxes 975 to 985 — Declaration
The name (975), date (980) and status (985) of the person declaring the return is correct and complete — in Tax Optimiser, the accepting director chosen on the Send Tax Return step.
Related: Submitting to HMRC and Companies House · Corporation Tax for charities and CASCs (CT600E).