Calculator methodology
Every formula behind the Your First Home, Shared Ownership and stamp duty calculators, every number that can change, its status and its source, and what the calculators leave out.
Register last reviewed 27 September 2026. 17 confirmed in current law or guidance, 6 announced by the government with final rules due at the Budget, 4 expected but not yet detailed, 15 working assumptions, 2 unknown.
The assumptions register
Every number the calculators use that could change is listed here with its status and source. This table is generated from the same file the calculators read, so it cannot drift from what the tools actually do. Status meanings: Confirmed in law or official guidance now, or published data from the named source such as a house price index, checked on the date shown; Announced, rules due at Budget stated in the government's announcement of 26 September 2026, with the final rules due at the Budget on 28 October 2026; Expected, detail to come the government has said the rule will exist but not what the figure is; Assumed our working assumption, usually the Help to Buy rules; Not yet known no information yet.
Your First Home scheme
Mortgage defaults
| What | Value used | Status | Source | Checked |
|---|---|---|---|---|
| Default mortgage rate (scheme mortgage, about 77.5% loan-to-value)Default only; the user can change it. Review monthly. Tracker rates were lower (about 4.2%) but variable. | 5% | Assumed | HomeOwners Alliance first-time buyer rate table, 26 Sept 2026: best fixed rates at 80% loan-to-value from lenders operating across England were Halifax 5.00% (two-year) and 5.02% (five-year); Barclays 5.03% (five-year). A Your First Home mortgage is 77.5% of the price, so it sits in the 75% to 80% band. | 27 September 2026 |
| Default rate for the 95% comparisonDefault for the 95% mortgage comparison and for Shared Ownership mortgages taken with a 5% deposit on the share. Review monthly. | 5.55% | Assumed | HomeOwners Alliance first-time buyer rate table, 26 Sept 2026: best 95% LTV fixed rates were NatWest 5.54% (five-year) and Bank of Ireland 5.57% (two-year) | 27 September 2026 |
| Default rate for a 90% mortgage (buying outright with 10% down)Used for the "buying outright with a 10% deposit" comparison row. Review monthly. | 5.15% | Assumed | HomeOwners Alliance first-time buyer rate table, 26 Sept 2026: best 90% LTV fixed rates from lenders operating across England were Halifax 5.13% (two-year) and 5.14% (five-year), Nationwide 5.18% | 27 September 2026 |
| Default mortgage term (years) | 25 | Assumed | Common first-time buyer mortgage term | 26 September 2026 |
| Affordability multiple (times income)Rule of thumb only. Lenders assess affordability individually. | 4.5 | Assumed | Typical lender cap of 4.5 times household income; some lenders go to 5 or 5.5 times | 26 September 2026 |
| Deposit used for the ordinary-mortgage comparison | 5% | Assumed | The smallest deposit most lenders accept without a scheme is 5% (a 95% mortgage), so that is what the scheme is compared with | 27 September 2026 |
Stamp duty (England and Northern Ireland)
| What | Value used | Status | Source | Checked |
|---|---|---|---|---|
| Standard stamp duty bands | 0% up to £125,000; 2% up to £250,000; 5% up to £925,000; 10% up to £1,500,000; 12% above that | Confirmed | GOV.UK, Stamp Duty Land Tax residential property rates | 26 September 2026 |
| First-time buyer relief | nothing to pay up to £300,000, then 5%, on the part up to £500,000, no relief above £500,000 | Confirmed | GOV.UK: first-time buyers pay no SDLT up to £300,000 and 5% on the portion from £300,001 to £500,000; no relief above £500,000 | 26 September 2026 |
Lifetime ISA
| What | Value used | Status | Source | Checked |
|---|---|---|---|---|
| Lifetime ISA annual limit | £4,000 | Confirmed | GOV.UK Lifetime ISA: up to £4,000 each year until you are 50 | 26 September 2026 |
| Lifetime ISA bonus | 25% | Confirmed | GOV.UK Lifetime ISA: 25% bonus, up to £1,000 a year | 26 September 2026 |
| Open a Lifetime ISA before age | 40 | Confirmed | GOV.UK Lifetime ISA: first payment before you are 40 | 26 September 2026 |
| Lifetime ISA home price limit | £450,000 | Confirmed | GOV.UK Lifetime ISA, withdrawing money: "the property costs £450,000 or less" (verified 26 Sept 2026) | 26 September 2026 |
| Lifetime ISA withdrawal charge | 25% | Confirmed | GOV.UK Lifetime ISA, withdrawing money: "You'll pay a withdrawal charge of 25%" (verified 26 Sept 2026) | 26 September 2026 |
| Lifetime ISA months open before buying | 12 | Confirmed | GOV.UK Lifetime ISA, withdrawing money: buy "at least 12 months after you make your first payment" (verified 26 Sept 2026) | 26 September 2026 |
Shared Ownership
Market figures
| What | Value used | Status | Source | Checked |
|---|---|---|---|---|
| Average UK house price | £298,468 | Confirmed | Lloyds House Price Index (formerly Halifax), August 2026 | 26 September 2026 |
| Example starter home price | £230,000 | Confirmed | Rightmove's average UK starter home price, the worked example used in press reports of the announcement ("With the average UK starter home costing £230,000 according to Rightmove, buyers could provide a £5,750 deposit and access up to £46,000 in loan support", The Independent, 26 Sept 2026). The GOV.UK release itself gives no example. | 27 September 2026 |
| Assumed inflation (CPI) | 2% | Assumed | Bank of England inflation target | 26 September 2026 |
The formulas
The worked examples below are computed by the same code the calculators run, at the example price of £230,000, so they change whenever the register changes.
Monthly mortgage payment
The standard repayment mortgage formula: monthly payment = loan × i ÷ (1 − (1 + i) to the power of −N), where i is the annual rate divided by 12 and N is the number of monthly payments. At a zero rate the payment is simply the loan divided by the number of months. Example: £178,250 at 5% over 25 years gives £1,042.03 a month.
The Your First Home split
Deposit = price × deposit percentage (minimum 2.5%). Equity loan = price × 20%. Mortgage = price minus deposit minus loan. Loan-to-value = mortgage ÷ price. On £230,000: deposit £5,750, loan £46,000, mortgage £178,250, loan-to-value 77.5%.
The 95% mortgage comparison
The same price with a 5% deposit and a 95% mortgage at the high loan-to-value default rate (5.55%). We also show the payment at the scheme rate so you can see how much of the saving comes from the smaller mortgage and how much from the cheaper rate. Example: £218,500 at 5.55% over 25 years is £1,348.31 a month, £306.28 more than the scheme mortgage; at the same 5% it would be £1,277.33, a gap of £235.30.
Interest after the interest-free period (assumption)
Years 1 to 5: no interest. Year 6: 1.75% of the amount borrowed, charged monthly (£67.08 a month on a £46,000 loan). Each later year the rate is multiplied by (1 + inflation + 2%), with inflation assumed at 2%, so 1.75% becomes 1.82%, and so on. Interest does not reduce the loan. This is the Help to Buy rule for loans taken from 2021 to 2023 (earlier Help to Buy loans used RPI plus 1%); the government has not published the Your First Home rule.
Repaying the equity loan (expected to follow Help to Buy)
Repayment = the loan's percentage of the original price (20%) × the home's value at the time of repayment. Scenarios show the value falling 10%, unchanged, rising 10% and rising 25%, plus a custom growth rate compounded yearly: value = price × (1 + growth) to the power of years. Help to Buy required a surveyor's valuation and a £200 fee for part or full repayment; we assume the same.
Affordability (rule of thumb)
Income needed = mortgage ÷ 4.5 (£178,250 ÷ 4.5 = £39,611). Maximum mortgage from income = income × 4.5; maximum price under the scheme = that mortgage ÷ (1 − deposit % − loan %). Lenders assess affordability individually; this is a guide only.
Saving the deposit
Months = deposit ÷ monthly saving, rounded up, ignoring interest on savings. With a Lifetime ISA, the first £333 of each month's saving earns the 25% bonus, so effective saving = saving + min(saving, £333) × 25%. A Lifetime ISA can only be used for a purchase 12 months after the first payment in, so the "with a Lifetime ISA" figure is never shorter than that. Example at £300 a month: 20 months without the bonus, 16 with it.
Stamp duty
Standard bands: 0% up to £125,000; 2% up to £250,000; 5% up to £925,000; 10% up to £1,500,000; 12% above that. First-time buyer relief on homes up to £500,000: nothing on the first £300,000, 5% on the part from there to £500,000. Above £500,000 the standard bands apply with no relief. The tax is rounded down to the nearest pound, as HMRC does. England and Northern Ireland only.
Shared Ownership
Share price = full value × share. Deposit = share price × deposit % (default 5%). Mortgage = share price minus deposit, paid by the mortgage formula above at the default rate for a 95% mortgage (5.55%). Rent = full value × (1 − share) × rent % (default 2.75% a year, the cap being 3%) ÷ 12. Total monthly = mortgage payment + rent + service charge. Staircasing cost = current full value × extra share, capped at the share the landlord still owns. Example on a £300,000 home at a 40% share: share £120,000, deposit £6,000, mortgage £114,000 costing £703.47 a month, rent £412.50, service charge £150, total £1,265.97.
Rounding and limits
Calculations are unrounded; displayed figures are rounded to the nearest pound (interest to the nearest penny, stamp duty down to the pound). Prices accepted from £50,000 to £2,000,000 (the slider stops at £800,000; higher prices can be typed); deposits from 2.5% to 50%; equity loans from 5% to 20% (Help to Buy allowed smaller loans, so the option is there in case the rules do); rates from 0% to 15%; terms from 5 to 40 years; Shared Ownership shares from 10% to 75%; stamp duty prices up to £5,000,000. Values outside these limits are replaced by the nearest limit and the calculator says so.
What the calculators leave out
- Buildings insurance, council tax, estate or service charges (except the Shared Ownership service charge you enter), utilities and maintenance.
- Mortgage fees, valuation fees, legal fees and the scheme's own administration fees.
- Interest-only mortgages, offset mortgages and variable rates over the term.
- Whether a lender will actually lend to you, and which lenders will join the scheme.
- The income cap and price caps, until they are announced.
- Tax on savings, and interest earned on savings while you build a deposit.
Testing
Every formula has automated tests against worked examples computed independently, and the test suite runs as part of every build. An independent recomputation of every figure on the site (over a thousand of them) was carried out before launch and is recorded in the project's testing log, along with the corrections it produced. If you find a result you believe is wrong, tell us: we will check it, fix it and record the change below.
Change log
- 27 September 2026: default scheme mortgage rate changed from 4.5% to 5% after an independent review found 4.5% was below every fixed rate in our source table; Shared Ownership mortgages now default to the 95% loan-to-value rate (5.55%) because the default deposit is 5% of the share; the Lifetime ISA 12-month rule applied to the savings timeline; stamp duty rounded down to the pound as HMRC does; falling house prices can now be entered as a minus figure.
- 27 September 2026: first version, built on the government announcement of 26 September 2026 and the Help to Buy rules as working assumptions.
Figures are illustrations, not quotes or advice. A mortgage is a loan secured on your home. Your home may be repossessed if you do not keep up repayments on your mortgage.