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Shared ownership calculator guide (UK): inputs, assumptions, and worked examples

A UK guide to using the Shared ownership calculator: what each input means (share, deposit, rent rate, service charge), what’s assumed, and 3 worked examples.

Published: 20/05/2026 • Last verified: 20/05/2026

Summary

Shared ownership monthly costs usually have three parts:

  1. A repayment mortgage payment on the share you’re buying
  2. Rent on the share you don’t own yet
  3. Service charges (common for flats; sometimes also estate charges)

The calculator estimates your monthly total using your inputs and a simple set of assumptions. Use it here:

Key terms (quick definitions)

  • Staircasing: buying more shares later, usually reducing rent as your share increases. See: Staircasing.
  • Leasehold: shared ownership homes are leasehold; your lease sets rules and costs. See: Leasehold.
  • Service charge: monthly/annual cost for communal areas/building management. See: Service charge.

How it works

The shared ownership calculator uses these inputs:

1) Property value

The full market value of the home.

2) Share percentage

The percentage you’re buying now (for example 25%, 40%, 50%).

3) Deposit percentage (of the share value)

The deposit is calculated on the share you’re buying, not the full property value.

4) Rent rate (annual % on the unsold share)

This is an annual rate applied to the value of the remaining share (the part you don’t own yet). Providers and leases vary, and rent-review rules differ depending on lease terms and date.

5) Service charge (monthly)

Service charges are an input because they vary hugely by building and location.

6) Mortgage rate and term

The calculator assumes a standard repayment mortgage on:

  • the share value minus your deposit

Outputs (what you get back)

  • share value and remaining value
  • deposit and mortgage amount
  • monthly mortgage payment, monthly rent, monthly service charge
  • total monthly cost (sum of the three monthly components)

Worked examples

Example 1: A typical “starter” share

Assume:

  • Property value: £300,000
  • Share: 40% → share value £120,000, remaining value £180,000
  • Deposit: 10% of share → £12,000
  • Mortgage amount: £108,000
  • Mortgage rate/term: 5.00% over 30 years
  • Rent rate: 2.75% on remaining share
  • Service charge: £150/month

Rent:

  • Annual rent = (£180,000 × 2.75% = £4,950)
  • Monthly rent = (£4,950 ÷ 12 = £412.50)

Mortgage payment (repayment): £579.77/month (calculator assumption).

Total monthly cost:

  • (£579.77 + £412.50 + £150 = £1,142.27/month)

Example 2: When service charges dominate the monthly cost

Assume:

  • Property value: £450,000
  • Share: 25% → share value £112,500, remaining value £337,500
  • Deposit: 5% of share → £5,625
  • Mortgage amount: £106,875
  • Mortgage rate/term: 4.50% over 35 years
  • Rent rate: 3.00% on remaining share
  • Service charge: £350/month

Rent:

  • Annual = (£337,500 × 3.00% = £10,125)
  • Monthly = (£10,125 ÷ 12 = £843.75)

Mortgage payment: £505.79/month.

Total monthly cost:

  • (£505.79 + £843.75 + £350 = £1,699.54/month)

Even with a smaller share, high service charges can make the monthly total higher than people expect.

Example 3: No mortgage (100% deposit on the share)

Assume:

  • Property value: £250,000
  • Share: 50% → share value £125,000, remaining value £125,000
  • Deposit: 100% of share → mortgage amount £0
  • Rent rate: 2.75% on remaining share
  • Service charge: £100/month

Rent:

  • Annual = (£125,000 × 2.75% = £3,437.50)
  • Monthly = (£3,437.50 ÷ 12 = £286.46) (rounded)

Total monthly cost:

  • (£0 + £286.46 + £100 = £386.46/month)

Common mistakes

  • Using the deposit % as if it’s of the full property value (the deposit is usually on your share).
  • Guessing the rent rate instead of using provider/lease figures.
  • Forgetting service charges (or using an unrealistically low number).
  • Treating the output as a guarantee (it’s a model; your lender/provider terms apply).
  • Ignoring how rent reviews can change the rent component over time.
  • Not running “what if rates rise?” scenarios for the mortgage payment.

What to do next

FAQ
What rent rate should I use in the calculator?
Your lease and provider set the rent and the rent-review rules. GOV.UK explains how shared ownership rent is set and reviewed; use your provider’s figures for accurate results.
Does staircasing reduce rent straight away?
In principle, as you buy more shares, rent is charged on a smaller remaining share. Admin and timing depend on your lease and provider.
Are service charges included in shared ownership?
Often, yes (especially for flats). Service charges can change over time and should be included in your monthly-cost planning.
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