function calculateSharedOwnership() {
var fullValue = parseFloat(document.getElementById('fullValue').value);
var sharePercent = parseFloat(document.getElementById('sharePercent').value);
var deposit = parseFloat(document.getElementById('deposit').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var termYears = parseFloat(document.getElementById('termYears').value);
var rentRate = parseFloat(document.getElementById('rentRate').value);
var serviceCharge = parseFloat(document.getElementById('serviceCharge').value);
if (isNaN(fullValue) || isNaN(sharePercent) || isNaN(deposit)) {
alert("Please fill in the property details correctly.");
return;
}
// Calculations
var shareValue = fullValue * (sharePercent / 100);
var unownedValue = fullValue – shareValue;
var mortgageAmount = shareValue – deposit;
// Monthly Rent Calculation
var monthlyRent = (unownedValue * (rentRate / 100)) / 12;
// Monthly Mortgage Calculation (Amortization formula)
var monthlyInterest = (interestRate / 100) / 12;
var numberOfPayments = termYears * 12;
var monthlyMortgage = 0;
if (mortgageAmount > 0) {
if (monthlyInterest === 0) {
monthlyMortgage = mortgageAmount / numberOfPayments;
} else {
monthlyMortgage = mortgageAmount * (monthlyInterest * Math.pow(1 + monthlyInterest, numberOfPayments)) / (Math.pow(1 + monthlyInterest, numberOfPayments) – 1);
}
}
var totalMonthly = monthlyMortgage + monthlyRent + serviceCharge;
// Format results
document.getElementById('resShareValue').innerText = "£" + shareValue.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resMortgageAmt').innerText = "£" + mortgageAmount.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resMonthlyMortgage').innerText = "£" + monthlyMortgage.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resMonthlyRent').innerText = "£" + monthlyRent.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resMonthlyService').innerText = "£" + serviceCharge.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resTotalMonthly').innerText = "£" + totalMonthly.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resultsArea').style.display = 'block';
}
Understanding Shared Ownership Costs
Shared Ownership is a government-backed scheme designed to help people get onto the property ladder. Instead of buying a property outright, you purchase a share of it (usually between 10% and 75%) and pay rent on the remaining portion held by a housing association.
How to Use the Shared Ownership Calculator
To get an accurate estimate of your monthly commitments, you need to input several variables that differ from a standard mortgage calculation:
Full Property Value: The total market value of the home if bought 100% outright.
Share Percentage: The portion of the property you are purchasing (e.g., 25%).
Cash Deposit: The amount of money you are putting down upfront. This only needs to be a percentage of the share you are buying, not the full property value.
Annual Rent Rate: Typically, housing associations charge around 2.75% of the unowned value per year.
Service Charge: Most shared ownership properties are leasehold flats or houses that require a monthly maintenance fee.
The Calculation Formula
Our calculator uses three distinct mathematical steps to determine your total monthly outflow:
Mortgage Payment: Calculated using the standard amortization formula based on your share value minus your deposit.
Rent Calculation:(Unowned Share Value × Rent Rate) / 12 months.
Total Sum: The addition of your mortgage, rent, and monthly service charges.
Example Scenario
Imagine a property valued at £300,000. You decide to buy a 25% share (£75,000).
Deposit: £7,500 (10% of the share).
Mortgage: £67,500 at 5.5% interest over 25 years = £414.50/month.
Rent: 2.75% of the £225,000 unowned share = £515.63/month.
Service Charge:£150.00/month.
Total Monthly Cost:£1,080.13.
Benefits of Shared Ownership
The primary benefit is the significantly lower deposit requirement. In the example above, a 10% deposit on the full property would be £30,000, whereas the shared ownership deposit is only £7,500. This makes homeownership accessible much earlier for first-time buyers and those with lower household incomes.
Expert Tip: Remember that as a shared owner, you are responsible for 100% of the maintenance costs of the property, regardless of the size of the share you own. Always factor in a "repair fund" in your monthly budget.