Shared Ownership Calculator United Kingdom

Shared ownership is a part-buy, part-rent scheme that lets you buy a home with a much smaller mortgage and deposit than buying outright. You buy a share of a home from a housing association and pay rent on the rest. Because the monthly cost is split between a mortgage and rent, the share price alone tells you little about what you will pay. This calculator puts the two together.

How shared ownership is priced

Two costs move in opposite directions. The share you buy is funded with a repayment mortgage, so a larger share means a bigger loan and mortgage payment. The share you do not buy is rented from the housing association, so the smaller your share, the more rent you pay. Rent is an annual percentage of the unowned value, often around 2.75%, added to the mortgage. The total is what matters, and it does not change in a straight line as you change the share.

You need a deposit only on the share you buy, not the full property value, so the cash needed to start is much lower: a 10% deposit on a 40% share of a £300,000 home is £12,000, not £30,000. That lower entry cost is the main reason people choose shared ownership, and the rent is the price of living in a home you do not yet fully own.

Working out the mortgage

The mortgage is a standard repayment loan on the value of your share, less the deposit. On a £300,000 home a 40% share is worth £120,000; with a £6,000 deposit you borrow £114,000. The monthly repayment depends on the interest rate and term, using the usual formula that spreads the loan and interest evenly over every month. A longer term lowers the monthly payment but increases the total interest.

Working out the rent

Rent applies to the part you do not own. In the same example, 60% is unowned, worth £180,000. At 2.75% a year that is £4,950 a year, or £412.50 a month. The rent keeps the starting cost down, but it builds no equity, so it behaves like renting. Rents usually rise each year in line with an inflation measure plus a small margin, so treat this figure as a starting point.

A worked example

Take a £300,000 home, a 40% share, a £6,000 deposit, a 5% mortgage over 300 months (25 years) and a 2.75% rent rate. The £114,000 mortgage costs about £666.43 a month and the rent on the £180,000 unowned share £412.50, a total of about £1,078.93 a month. With a 50% share and the same deposit, the mortgage rises to about £841.81 and the rent falls to £343.75, a total of about £1,185.56.

Staircasing

You can buy more shares over time, which increases your mortgage but reduces or eventually removes the rent. Many buyers aim to reach full ownership so the rent stops. Running different share percentages through the calculator shows how the balance between mortgage and rent shifts as you buy more.

Staircasing has costs the calculator does not include. Each extra share is priced at the current market value, so if the home has gone up in value the next share costs more, and you usually pay valuation, legal and mortgage fees each time. That is why some buyers staircase in a few larger steps, and why it is worth weighing the monthly rent saved against the one-off cost of buying the extra share.

Common mistakes to avoid

• Comparing shared ownership on the mortgage payment alone and ignoring the rent.
• Leaving out the service charge and any ground rent, which on a new-build flat can be as large as the rent and are not shown here.
• Assuming the smallest share is always cheapest. Test a few shares at your own rate and term.
• Forgetting repairs and maintenance. Under most leases you are responsible for upkeep of the whole home, not just your share.

How to read your result

Treat the total monthly figure as the minimum housing cost, and add your expected service charge and a maintenance allowance before judging affordability. Compare a few share percentages side by side. If the total is close to what a mortgage on the whole property would cost, consider buying a larger share or buying outright.

Assumptions and limitations

This is an estimate for planning and comparison. It assumes a capital and interest repayment mortgage at a fixed rate for the whole term, and does not model interest-only mortgages, rate changes or annual rent increases. It excludes service charges, ground rent, buildings insurance, Stamp Duty and staircasing costs, all of which can be substantial. Rent is a flat annual percentage of the unowned share for the current period. Check the housing association’s offer and speak to a mortgage adviser before committing.

About This Shared Ownership Calculator

Use this shared ownership calculator to get an instant estimate based on the figures you enter. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.