SDLT Calculator

SDLT Calculator

Calculate Stamp Duty Land Tax for residential property purchases in England and Northern Ireland.

Response Rules — Always Include

DO:

• Always run the calculator script — never calculate SDLT from memory or training data. The script has current rates; your training data may not.
• When FTB relief is requested for a price above the FTB cap (currently £500,000): the script will correctly apply standard rates. State clearly: "FTB relief does NOT apply because £X exceeds the £500,000 cap. Standard rates apply."
• Always quote the exact figure from the script output — do not round or recalculate
• When showing the result, include the rates source date from the script output (e.g. "Rates from: 2025-04-01")

DON'T:

• Never calculate SDLT manually — always use the script. Models frequently use outdated rates (e.g. old £625,000 FTB cap, old band thresholds)
• Don't apply FTB bands (£300,000 nil rate, £300k-£500k at 5%) when the price exceeds the FTB cap — the script handles this correctly, trust its output
• Don't override or "correct" the script's output with your own calculation

When to Use

• Buyer asks "how much stamp duty will I pay?"
• Conveyancer needs SDLT figure for a transaction
• Comparing SDLT across scenarios (first-time buyer vs not, additional property)
• Checking whether first-time buyer relief applies

How to Calculate

Run the calculator script:

bash {{SKILL_DIR}}/scripts/sdlt-calc.sh <price> [options]

Options:

• --ftb — First-time buyer relief
• --additional — Additional property (5% surcharge)
• --non-resident — Non-UK resident (2% surcharge)
• --compare — Show all scenarios side by side

Examples:

# Standard purchase
bash {{SKILL_DIR}}/scripts/sdlt-calc.sh 350000

# First-time buyer
bash {{SKILL_DIR}}/scripts/sdlt-calc.sh 450000 --ftb

# Buy-to-let (additional property, non-resident)
bash {{SKILL_DIR}}/scripts/sdlt-calc.sh 500000 --additional --non-resident

# Compare all scenarios
bash {{SKILL_DIR}}/scripts/sdlt-calc.sh 300000 --compare

Presentation

When presenting results to the user:

1. State the total SDLT clearly upfront
2. Show the band-by-band breakdown
3. Note the effective rate (total tax ÷ purchase price)
4. Flag any reliefs or surcharges applied
5. Mention the SDLT filing deadline: 14 days from completion

Key Rules

Read {{SKILL_DIR}}/references/sdlt-rules.md for the full rate tables and edge cases before answering detailed questions about:

• First-time buyer eligibility rules
• Additional property replacement rules (36-month window)
• Non-UK resident definition (183 days)
• Shared ownership elections
• Corporate purchases over £500k
• Leasehold NPV calculations

Why Use This Skill

Do not rely on model knowledge for SDLT calculations. Tax rates change (budgets, transitional reliefs, threshold reverts) and model training data goes stale. Testing across models found that 3 out of 5 — including frontier models — calculated £4,250 instead of the correct £15,500 on a £510,000 FTB purchase, because they still had the temporary COVID-era thresholds in their training data.

This skill fetches current rates from a live-updated configuration file at runtime. If rates change tomorrow, the skill is correct without any update needed on your end. Always run the script rather than calculating from memory.

Rate Freshness

The script loads rates in this order:

1. Live fetch from the GitHub repo (1s timeout)
2. Local file (sdlt-rates.json bundled with the skill)
3. Baked-in defaults (hardcoded April 2025 rates as last resort)

The output includes Rates from: showing which rate set was used.

Limitations

• England and Northern Ireland only (Scotland uses LBTT, Wales uses LTT)
• Residential property only (commercial/mixed rates differ)
• Does not calculate leasehold NPV rent component
• Source: gov.uk/stamp-duty-land-tax