Shared ownership calculator: monthly cost of your share

How to use this shared ownership calculator

This shared ownership calculator turns the full value of a home and the share you want into one monthly figure: the mortgage on your share, the rent on the landlord’s share and the service charge, added together. Type the full market value from the listing, move the slider to the share you want, and the results update as you type. GOV.UK says the share is usually between 25% and 75%, with 10% available on some homes (GOV.UK), so the slider runs from 10% to 75%.

Open “More options” to change the assumptions:

• Deposit, as a percentage of your share. GOV.UK says it is usually between 5% and 10% of the share you are buying (GOV.UK). The default is 5%.
• Mortgage interest rate and term. The defaults are 5.55% over 25 years: a 5% deposit on the share means a 95% mortgage on it, and 5.55% was the best 95% fixed rate in the HomeOwners Alliance rate table on 26 September 2026. Put in your own quote; a bigger deposit on the share earns a cheaper rate.
• Rent on the unsold share. The default is 2.75% a year, the rate GOV.UK says most landlords charge, under a 3% limit on new-build homes (GOV.UK); Share to Buy, the listing portal, says the same (Share to Buy). Older homes and resales can differ, so use the figure in the listing.
• Service charge a month. Flats almost always have one; houses on new estates often have an estate charge instead. The default of £150 is a placeholder, not a fact about any home.
• Extra share to buy later. This prices a staircasing purchase at today’s value.

Press “Copy a link to this result” to save or share your numbers. Nothing you type is stored.

What the results mean

• Price of your share is the full value multiplied by your percentage. This, not the full price, is what you are buying.
• Deposit is your deposit percentage applied to the share price. On a 40% share of a £300,000 home at 5%, that is £6,000, not the £15,000 a 5% deposit on the whole home would need.
• Mortgage on your share is the share price minus the deposit. The lender lends on your share only and takes a charge over your lease.
• Mortgage payment is the monthly cost of a repayment mortgage at the rate and term you set. The calculator methodology shows the formula.
• Rent on the part you do not own is the yearly rent percentage applied to the landlord’s share of the value, divided by 12. Rent is money you never get back; it is the price of living in the part of the home you have not bought yet.
• Service charge is what the landlord or managing agent charges to clean, maintain and insure shared parts of the building or estate (GOV.UK). You pay 100% of it whatever share you own. Some homes also have a management fee and payments into a repairs reserve fund; add them to this line.
• Total monthly cost is the three added together. It is the number a mortgage adviser will test against your income.
• The same home other ways compares your share with buying the same home under Your First Home and with buying outright with a 10% deposit.
• Buying more later shows what an extra share would cost at today’s value, before valuation, legal and administration fees; the staircasing page lists those.

Worked example: a £300,000 home with a 40% share

Here is the default calculation, step by step, so you can see where each number comes from.

Of the £1,266, only the £703 mortgage payment builds anything for you, and in the early years most of that is interest. The £413 rent and the £150 service charge, about £563 a month or £6,750 a year, are the cost of the arrangement. Over five years that is close to £34,000 before any rent increases.

What changes the monthly cost most?

The share you choose. A smaller share means a smaller deposit and mortgage but a bigger rent bill, because the landlord keeps more of the home. All figures below use the default settings on a £300,000 home.

Read the rent column against the mortgage column. At 10%, rent is about three and a half times the mortgage payment; at 75% it is about an eighth of it. The cheapest monthly total is also the one where most of your money disappears.

After the share, the biggest levers are the mortgage rate, where a difference of one percentage point on £114,000 over 25 years moves the payment by roughly £70 a month, and the service charge, which you should take from the listing and the last three years of accounts rather than guess.

Is this a shared ownership affordability calculator?

It is the first half of one. Lenders limit how many mortgages they grant at 4.5 times income or more, because a Bank of England rule keeps such loans to 15% of new lending across the market; since July 2025 an individual lender may go above 15% while the market as a whole stays within it (Bank of England), so the mortgage on your share still needs to sit comfortably under that multiple for most lenders. The lender then treats the rent and service charge as committed spending. Before you get that far, the landlord checks you can afford the home with a mortgage adviser (GOV.UK). The mortgages page explains both checks.

How does it compare with Your First Home?

The comparison box prices the same home under Your First Home, the new government equity loan: a 2.5% deposit, a 20% government equity loan and a mortgage for the remaining 77.5% (GOV.UK). On the £300,000 example that is a £7,500 deposit and a £232,500 mortgage costing about £1,359 a month at 5% (the best rate for a mortgage of that size against the price), with no rent.

Shared ownership at 40% is cheaper by about £93 a month once its £150 service charge is counted (about £243 before it) and needs £1,500 less up front, but you own 40% of the home rather than all of it, and £563 of your monthly payment is rent and charges. Your First Home costs more each month, but the payment builds equity in the whole home; the price is repaying 20% of the home’s value when you sell. Your First Home only works on a new build from a participating developer, and its full rules, including the income cap and price caps, come at the Budget on 28 October 2026. The Your First Home vs Shared Ownership page sets the two side by side, and the is it worth it page runs the five-year figures.

What the calculator does not do

It does not check your eligibility; the eligibility page covers the £80,000 income cap (£90,000 in London). It does not include stamp duty, buildings insurance, council tax, ground rent where a lease has one, or the valuation, legal and administration fees of buying more shares. It shows the starting rent, not the rent after the yearly reviews that GOV.UK says only ever move it up (GOV.UK). And it prices staircasing at today’s value, whereas GOV.UK says extra shares of 5% or more are priced at a surveyor’s valuation on the day, and 1% shares by the House Price Index (GOV.UK).

What to do next

• Put in the real figures from a listing: full value, the share on offer, the rent and the service charge.
• Add 3% a year to the rent in your head and ask whether the total still works in year five.
• Compare the result with the Your First Home row, then read the Your First Home vs Shared Ownership page.
• Take the total monthly figure to a mortgage adviser, not just the mortgage payment.