How to Build a Stamp Duty Calculator

interface StampDutyBand {
  min: number;
  max: number;  // Infinity for the top band
  rate: number; // as decimal, e.g., 0.05 for 5%
  }

  interface BuyerProfile {
  purchasePrice: number;
  isFirstTimeBuyer: boolean;
  isAdditionalProperty: boolean; // second home or buy-to-let
  isNonResident: boolean;
  country: "england" | "scotland" | "wales";
  }

  // England/NI SDLT rates 2025-26 (standard residential)
  const SDLT_STANDARD: StampDutyBand[] = [
  { min: 0, max: 125000, rate: 0 },
  { min: 125001, max: 250000, rate: 0.02 },
  { min: 250001, max: 925000, rate: 0.05 },
  { min: 925001, max: 1500000, rate: 0.10 },
  { min: 1500001, max: Infinity, rate: 0.12 },
  ];

  const SDLT_FIRST_TIME: StampDutyBand[] = [
  { min: 0, max: 300000, rate: 0 },
  { min: 300001, max: 500000, rate: 0.05 },
  // If price > 500,000, first-time relief does not apply, fall back to standard
  ];

  const ADDITIONAL_PROPERTY_SURCHARGE = 0.05; // 5% surcharge
  const NON_RESIDENT_SURCHARGE = 0.02;        // 2% surcharge

  // Scotland LBTT rates
  const LBTT_STANDARD: StampDutyBand[] = [
  { min: 0, max: 145000, rate: 0 },
  { min: 145001, max: 250000, rate: 0.02 },
  { min: 250001, max: 325000, rate: 0.05 },
  { min: 325001, max: 750000, rate: 0.10 },
  { min: 750001, max: Infinity, rate: 0.12 },
  ];

  // Wales LTT rates
  const LTT_STANDARD: StampDutyBand[] = [
  { min: 0, max: 225000, rate: 0 },
  { min: 225001, max: 400000, rate: 0.06 },
  { min: 400001, max: 750000, rate: 0.075 },
  { min: 750001, max: 1500000, rate: 0.10 },
  { min: 1500001, max: Infinity, rate: 0.12 },
  ];

  interface StampDutyResult {
  totalTax: number;
  effectiveRate: number;
  bandBreakdown: { band: string; taxableAmount: number; rate: number; tax: number }[];
  surcharges: { name: string; amount: number }[];
  totalWithSurcharges: number;
  }

  function calculateStampDuty(buyer: BuyerProfile): StampDutyResult {
  let bands: StampDutyBand[];

  // Select correct band table
  if (buyer.country === "scotland") {
    bands = LBTT_STANDARD;
  } else if (buyer.country === "wales") {
    bands = LTT_STANDARD;
  } else {
    // England/NI
    if (buyer.isFirstTimeBuyer && buyer.purchasePrice <= 500000) {
      bands = SDLT_FIRST_TIME;
    } else {
      bands = SDLT_STANDARD;
    }
  }

  // Calculate tiered tax
  const breakdown: StampDutyResult["bandBreakdown"] = [];
  let totalTax = 0;

  for (const band of bands) {
    if (buyer.purchasePrice <= band.min) break;
    const taxableInBand = Math.min(buyer.purchasePrice, band.max) - band.min;
    const taxForBand = Math.max(0, taxableInBand) * band.rate;
    totalTax += taxForBand;
    if (taxableInBand > 0) {
      breakdown.push({
        band: `${formatCurrency(band.min)} - ${band.max === Infinity ? "+" : formatCurrency(band.max)}`,
        taxableAmount: Math.max(0, taxableInBand),
        rate: band.rate,
        tax: taxForBand,
      });
    }
  }

  // Surcharges (applied on full price as additional % on each band)
  const surcharges: { name: string; amount: number }[] = [];
  if (buyer.isAdditionalProperty) {
    const surcharge = buyer.purchasePrice * ADDITIONAL_PROPERTY_SURCHARGE;
    surcharges.push({ name: "Additional Property Surcharge (5%)", amount: surcharge });
  }
  if (buyer.isNonResident && buyer.country === "england") {
    const surcharge = buyer.purchasePrice * NON_RESIDENT_SURCHARGE;
    surcharges.push({ name: "Non-UK Resident Surcharge (2%)", amount: surcharge });
  }

  const surchargeTotal = surcharges.reduce((sum, s) => sum + s.amount, 0);

  return {
    totalTax,
    effectiveRate: totalTax / buyer.purchasePrice,
    bandBreakdown: breakdown,
    surcharges,
    totalWithSurcharges: totalTax + surchargeTotal,
  },
  }

  function formatCurrency(amount: number): string {
  return new Intl.NumberFormat("en-GB", { style: "currency", currency: "GBP", maximumFractionDigits: 0 }).format(amount);
  }