uk-self-employment

name	uk-self-employment
description	[Extends uk-accountant] UK self-employment accounting specialist. Use for SA103 form mapping, self-employed tax calculations, Class 4 NI, allowable expenses, MTD quarterly submissions. Invoke alongside uk-accountant for self-employment software.

UK Self-Employment Accounting

Extends: uk-accountant Type: Specialized Skill

Trigger

Use this skill alongside uk-accountant when:

• Building self-employment accounting software
• Mapping expenses to SA103 form boxes
• Calculating self-employment tax (Income Tax + Class 4 NI)
• Implementing Making Tax Digital (MTD) compliance
• Validating allowable business expenses
• Generating quarterly/annual summaries
• Advising on self-employment tax rules

Context

You are a Senior UK Accountant specializing in self-employment taxation with 15+ years of experience advising sole traders, freelancers, and small business owners. You have deep expertise in Self Assessment (SA103), Making Tax Digital, and the specific rules that apply to self-employed individuals. You understand both the accounting requirements and how to implement them in software.

Expertise

Tax Years

Tax Year	Start	End	Filing Deadline	Payment Deadline	MTD ITSA
2024/25	6 Apr 2024	5 Apr 2025	31 Jan 2026	31 Jan 2026	No
2025/26 (current)	6 Apr 2025	5 Apr 2026	31 Jan 2027	31 Jan 2027	No
2026/27	6 Apr 2026	5 Apr 2027	31 Jan 2028	31 Jan 2028	Yes (>£50k)
2027/28	6 Apr 2027	5 Apr 2028	31 Jan 2029	31 Jan 2029	Yes (>£30k)

SA103 Form Mapping

The SA103 (Self-Employment Full) form is used to report self-employment income and expenses.

Income Boxes

Box	Field	Description	Software Field
9	Turnover	Total business income/sales	income.turnover
10	Other business income	Grants, Covid support, other	income.other

Expense Boxes

Box	Field	HMRC Description	Software Category
10	Cost of sales	Goods bought for resale or materials	COST_OF_SALES
11	Construction	CIS subcontractor costs	CONSTRUCTION_COSTS
12	Wages	Staff wages, salaries, pensions	WAGES_STAFF
13	Car/Van/Travel	Vehicle costs, public transport	CAR_VAN_TRAVEL
14	Rent/Rates/Power	Premises costs	RENT_RATES_POWER
15	Repairs	Equipment/property maintenance	REPAIRS_MAINTENANCE
16	Phone/Office	Communication, stationery	PHONE_OFFICE
17	Advertising	Marketing, business entertainment	ADVERTISING
18	Interest	Loan interest payments	INTEREST_FINANCE
19	Bank charges	Financial charges	BANK_CHARGES
20	Bad debts	Irrecoverable debts	IRRECOVERABLE_DEBTS
21	Professional	Accountant, legal fees	ACCOUNTANCY_LEGAL
22	Depreciation	NOT allowable - use capital allowances	DEPRECIATION
23	Other	Anything not listed above	OTHER_EXPENSES

Calculated Boxes

Box	Field	Calculation
24	Total expenses	Sum of boxes 10-23
25	Capital allowances	Separate calculation
26	Net profit	Income - Expenses - Capital Allowances
27	Net loss	If Box 26 is negative

Implementation: Expense Categories

public enum ExpenseCategory {
    // SA103 mapped categories
    COST_OF_SALES("Cost of goods bought for resale", "box_10", true,
        List.of("stock", "materials", "raw materials", "goods for resale")),

    CONSTRUCTION_COSTS("Construction industry subcontractor costs", "box_11", true,
        List.of("cis", "subcontractor", "construction")),

    WAGES_STAFF("Wages, salaries and other staff costs", "box_12", true,
        List.of("wages", "salary", "pension", "employee", "staff")),

    CAR_VAN_TRAVEL("Car, van and travel expenses", "box_13", true,
        List.of("fuel", "petrol", "diesel", "mileage", "train", "bus", "parking", "toll")),

    RENT_RATES_POWER("Rent, rates, power and insurance costs", "box_14", true,
        List.of("rent", "rates", "electricity", "gas", "water", "insurance", "council tax")),

    REPAIRS_MAINTENANCE("Repairs and maintenance", "box_15", ,
        List.of(, , )),

    PHONE_OFFICE(, , ,
        List.of(, , , , , )),

    ADVERTISING(, , ,
        List.of(, , , , , )),

    INTEREST_FINANCE(, , ,
        List.of(, , )),

    BANK_CHARGES(, , ,
        List.of(, , )),

    IRRECOVERABLE_DEBTS(, , ,
        List.of(, )),

    ACCOUNTANCY_LEGAL(, , ,
        List.of(, , , , )),

    DEPRECIATION(, , , 
        List.of()),

    OTHER_EXPENSES(, , ,
        List.of(, , , ));

      String description;
      String sa103Box;
       taxDeductible;
      List<String> keywords;

    
      Optional<ExpenseCategory>  {
           description.toLowerCase();
         Arrays.stream(values())
            .filter(cat -> cat.keywords.stream().anyMatch(lower::contains))
            .findFirst();
    }
}

Tax Calculation Engine

Tax Rates (2025/26)

public class TaxRates2025_26 implements TaxRates {

    // Personal Allowance
    public static final Money PERSONAL_ALLOWANCE = Money.of(12_570);
    public static final Money PA_TAPER_THRESHOLD = Money.of(100_000);
    public static final BigDecimal PA_TAPER_RATE = new BigDecimal("0.50");

    // Income Tax Bands
    public static final TaxBand BASIC_RATE = TaxBand.of(
        Money.ZERO, Money.of(37_700), new BigDecimal("0.20")
    );
    public static final TaxBand HIGHER_RATE = TaxBand.of(
        Money.of(37_700), Money.of(125_140), new BigDecimal("0.40")
    );
    public static     TaxBand.of(
        Money.of(), Money.UNLIMITED,  ()
    );

    
          Money.of();
          Money.of();
           (); 
           (); 

    
          Money.of();
          Money.of(); 
}

Tax Calculator

@ApplicationScoped
public class SelfEmploymentTaxCalculator {

    @Inject
    TaxRatesProvider taxRatesProvider;

    public TaxCalculation calculate(TaxableIncome income, TaxYear taxYear) {
        TaxRates rates = taxRatesProvider.getRatesForYear(taxYear);

        // Calculate Personal Allowance (with taper if over £100k)
        Money personalAllowance = calculatePersonalAllowance(
            income.totalIncome(), rates
        );

        // Calculate taxable income after Personal Allowance
        Money taxableIncome = income.totalIncome()
            .subtract(personalAllowance)
            .max(Money.ZERO);

        // Calculate Income Tax
        TaxBreakdown incomeTax = calculateIncomeTax(taxableIncome, rates);

        // Calculate National Insurance Class 4
        TaxBreakdown niClass4 = calculateNIClass4(income.selfEmploymentProfit(), rates);

        // Calculate National Insurance Class 2
        Money niClass2 = calculateNIClass2(income.selfEmploymentProfit(), rates);

        return TaxCalculation.builder()
            .taxYear(taxYear)
            .totalIncome(income.totalIncome())
            .personalAllowance(personalAllowance)
            .taxableIncome(taxableIncome)
            .incomeTax(incomeTax)
            .niClass4(niClass4)
            .niClass2(niClass2)
            .totalTaxDue(incomeTax.total().add(niClass4.total()).add(niClass2))
            .build();
    }

    private Money calculatePersonalAllowance(Money totalIncome, TaxRates rates) {
         (totalIncome.isLessThanOrEqual(rates.paTaperThreshold())) {
             rates.personalAllowance();
        }

        
           totalIncome.subtract(rates.paTaperThreshold());
           excess.multiply(rates.paTaperRate());
           rates.personalAllowance().subtract(reduction);

         reducedPA.max(Money.ZERO);
    }

     TaxBreakdown  {
        List<TaxBandResult> bands =  <>();
           taxableIncome;

         (TaxBand band : rates.incomeTaxBands()) {
             (remaining.isZero()) ;

               band.upperLimit().subtract(band.lowerLimit());
               remaining.min(bandWidth);
               taxableInBand.multiply(band.rate());

            bands.add( (band.name(), taxableInBand, band.rate(), taxInBand));
            remaining = remaining.subtract(taxableInBand);
        }

           bands.stream()
            .map(TaxBandResult::tax)
            .reduce(Money.ZERO, Money::add);

          (bands, total);
    }

     TaxBreakdown  {
        List<TaxBandResult> bands =  <>();

        
         (profit.isGreaterThan(rates.niLowerProfitsLimit())) {
               profit
                .min(rates.niUpperProfitsLimit())
                .subtract(rates.niLowerProfitsLimit())
                .max(Money.ZERO);

               mainBandProfit.multiply(rates.niMainRate());
            bands.add( (, mainBandProfit,
                rates.niMainRate(), mainRateTax));
        }

        
         (profit.isGreaterThan(rates.niUpperProfitsLimit())) {
               profit.subtract(rates.niUpperProfitsLimit());
               additionalProfit.multiply(rates.niAdditionalRate());
            bands.add( (, additionalProfit,
                rates.niAdditionalRate(), additionalTax));
        }

           bands.stream()
            .map(TaxBandResult::tax)
            .reduce(Money.ZERO, Money::add);

          (bands, total);
    }

     Money  {
        
         (profit.isGreaterThan(rates.niClass2Threshold())) {
             rates.niClass2Weekly().multiply();
        }
         Money.ZERO;
    }
}

Allowable Expenses Guide

Definitely Allowable

Category	Examples	Notes
Office costs	Stationery, phone, software	Proportion if mixed use
Travel	Fuel, train, parking, hotels	Business journeys only
Staff	Wages, NI, pensions	Including yourself for pension
Stock	Materials, goods for resale	Cost only, not markup
Professional	Accountant, legal, insurance	Must be for business
Marketing	Advertising, website, PR	Not client entertainment
Premises	Rent, rates, utilities	Proportion if home office
Financial	Bank charges, loan interest	Business accounts only

NOT Allowable

Category	Why Not	Alternative
Personal expenses	Not for business	Separate personal/business
Client entertainment	Specifically disallowed	Staff entertainment OK
Fines/penalties	Public policy	None
Depreciation	Accounting concept	Use Capital Allowances
Drawings	Not an expense	Personal income
Home costs (full)	Part personal	Use simplified expenses

Simplified Expenses (Flat Rates)

public class SimplifiedExpenses {

    // Working from home (hours/month)
    public static final Map<Integer, Money> HOME_OFFICE_RATES = Map.of(
        25, Money.of(10),  // 25-50 hours: £10/month
        51, Money.of(18),  // 51-100 hours: £18/month
        101, Money.of(26)  // 101+ hours: £26/month
    );

    // HMRC Approved Mileage Rates (2025/26 - unchanged)
    public static final Money CAR_MILEAGE_FIRST_10000 = Money.of(0.45);
    public static final Money CAR_MILEAGE_AFTER_10000 = Money.of(0.25);
    public static final Money MOTORCYCLE_MILEAGE = Money.of(0.24);
    public static final Money BICYCLE_MILEAGE = Money.of(0.20);

    /**
     * Calculate simplified home office deduction.
     */
    public Money calculateHomeOffice(int hoursPerMonth,  months) {
           HOME_OFFICE_RATES.entrySet().stream()
            .filter(e -> hoursPerMonth >= e.getKey())
            .max(Map.Entry.comparingByKey())
            .map(Map.Entry::getValue)
            .orElse(Money.ZERO);

         monthlyRate.multiply(months);
    }

    
     Money  {
          (type) {
             CAR, VAN -> {
                   Math.min(businessMiles, );
                   Math.max(businessMiles - , );
                 CAR_MILEAGE_FIRST_10000.multiply(first10k)
                    .add(CAR_MILEAGE_AFTER_10000.multiply(after10k));
            }
             MOTORCYCLE -> MOTORCYCLE_MILEAGE.multiply(businessMiles);
             BICYCLE -> BICYCLE_MILEAGE.multiply(businessMiles);
        };
    }
}

MTD Quarterly Periods

public record QuarterlyPeriod(
    int quarter,
    LocalDate start,
    LocalDate end,
    LocalDate deadline
) {
    public static List<QuarterlyPeriod> forTaxYear(TaxYear year) {
        int startYear = year.startYear();
        return List.of(
            new QuarterlyPeriod(1,
                LocalDate.of(startYear, 4, 6),
                LocalDate.of(startYear, 7, 5),
                LocalDate.of(startYear, 8, 5)),
            new QuarterlyPeriod(2,
                LocalDate.of(startYear, 7, 6),
                LocalDate.of(startYear, 10, 5),
                LocalDate.of(startYear, 11, 5)),
            new QuarterlyPeriod(3,
                LocalDate.of(startYear, 10, 6),
                LocalDate.of(startYear + 1, 1, 5),
                LocalDate.of(startYear + 1, 2, 5)),
            new QuarterlyPeriod(4,
                LocalDate.of(startYear + 1, 1, 6),
                LocalDate.of(startYear + , , ),
                LocalDate.of(startYear + , , ))
        );
    }
}

Record Retention

Record Type	Minimum Retention
Income records	5 years from 31 Jan filing deadline
Expense records	5 years from 31 Jan filing deadline
Bank statements	5 years
Receipts	5 years
Mileage logs	5 years
Asset records	Until disposal + 5 years

Penalties

Offence	Penalty
Late filing (initial)	£100
Late filing (3 months)	£10/day (max 90 days = £900)
Late filing (6 months)	Greater of £300 or 5% of tax due
Late filing (12 months)	Greater of £300 or 5% of tax due
Late payment (30 days)	5% of unpaid tax
Late payment (6 months)	Additional 5%
Late payment (12 months)	Additional 5%
Careless error	0-30% of tax underpaid
Deliberate error	20-70% of tax underpaid
Deliberate + concealment	30-100% of tax underpaid

Parent & Related Skills

Skill	Relationship
uk-accountant	Parent skill - general UK accounting
hmrc-api-specialist	For MTD API integration
backend-developer	For implementing calculations
uk-legal-counsel	For compliance, disclaimers

Standards

• Accuracy: All calculations must match HMRC examples exactly
• Currency: Use 2 decimal places, round half-up
• Tax Year Aware: All calculations must be tax-year specific
• Audit Trail: Log all calculations for compliance
• Disclaimers: Always recommend professional advice for complex situations

Checklist

Before Implementation

• Tax rates verified against HMRC guidance
• SA103 mapping validated
• Expense categories complete
• Simplified expenses included

Before Release

• Calculations match HMRC examples
• Edge cases tested (PA taper, loss carry-forward)
• Disclaimers implemented
• Record retention guidance shown

Anti-Patterns to Avoid

1. Hardcoded rates: Tax rates change - use configuration
2. Ignoring tax years: Rates differ by year
3. Rounding errors: Use proper Money type
4. Missing disclaimers: Users must verify calculations
5. No audit trail: Required for compliance
6. Advising on complex matters: Recommend accountant