If you’ve fallen down the rabbit hole of trying every lease extension calculator online and getting wildly different results, you’re not alone. Most calculators throw numbers at you without explaining where they came from, why the price shifts when your lease drops below 80 years, or why your freeholder’s quote seems suspiciously higher than every online estimate.
This guide clears the fog.
Using a mix of 2025 industry practice, surveyor-backed methods, and plain-English breakdowns, this article helps you:
- Understand how lease extension costs are truly calculated
- Predict a realistic price range using simple inputs
- Avoid the traps most leaseholders fall into
- Compare 90-year vs. 999-year extensions
- Learn whether a 110-year lease is “long enough.”
- See real examples — not vague theory
- Use a framework that works even if the calculator isn’t perfect
Let’s break it down step-by-step — without jargon, without scare tactics, and without the robotic tone most law websites use.
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How are lease extension costs calculated? |
What a Lease Extension Calculator Actually Does (and What It Can’t Do)
Every calculator is trying to estimate the premium — the lump sum you pay to your freeholder when you extend your lease.
The formula (simplified) includes:
- The current lease length
- Ground rent (and how it increases)
- Your property’s current market value
- The “relativity” of your lease (value drop as the lease shortens)
- Marriage value (if below 80 years)
- Deferment rate (industry standard)
But here’s the catch:
No calculator on the internet has access to the exact relativity tables and surveyor modelling used by tribunals.
Meaning: calculators give ballpark figures, not binding quotes.
Your goal as a homeowner?
Use a calculator as a signal — not gospel.
How Lease Extension Costs Are Calculated
Lease extension cost has 3 parts:
1. Loss of Ground Rent
You’re wiping out your old ground rent forever.
So the freeholder wants compensation for that.
Higher or escalating ground rent = higher premium.
2. Reversion Value
At the end of your lease, the freeholder would eventually get the flat back.
Extending the lease delays that date, so they charge for the delay.
3. Marriage Value (Only if Your Lease Is Under 80 Years)
This is the big-ticket item.
When your lease is under 80 years, extending it increases the property’s value.
By law, you split that value increase 50/50 with the freeholder.
Example:
- Flat worth now: £300,000
- After extension: £330,000
- Increase: £30,000
- Half owed to freeholder: £15,000 (marriage value)
This is why costs jump suddenly at 79 years.
The 5-Box Lease Extension Estimate Framework™
To get a reliable estimate, fill in these five boxes:
1️⃣ Property value (today)
2️⃣ Current lease length
3️⃣ Ground rent + how it rises
4️⃣ Local sale data for similar lease lengths
5️⃣ Whether you qualify for the 90-year statutory route
Plug these into any calculator — then average your results across 2–3 calculators.
You’ll be within 10–20% of what a surveyor will quote, which is considered very accurate in leasehold valuations.
Mini Case Study (2025 Example)
Flat value: £350,000
Lease: 72 years
Ground rent: £250 → doubles every 25 years
Average calculator outputs:
- MyLeasehold: £22,000–£28,000
- Connaught Law: £20,500–£25,000
- Brady Solicitors: £24,000–£30,000
Surveyor estimate for tribunal-level accuracy:
£26,000–£29,000
The difference range is normal — each calculator uses different relativity tables and assumptions.
Also Check: Council Tax Mansion Tax 2025: Who Pays & How Much
Statutory vs. Informal Lease Extensions
| Factor | Statutory (90 years, £0 ground rent) | Informal (variable) |
|---|---|---|
| Ground rent | Set to zero permanently | Freeholder may keep or increase it |
| Length added | +90 years | Negotiated (may be 99, 125, 250, 999 years) |
| Cost | Usually fairer | Sometimes cheaper upfront, more expensive long term |
| Legal protection | Strong | None |
| Risk | Low | Medium–High |
| Best choice | Most leaseholders | Only with expert advice |
If your freeholder pushes an informal deal quickly, have a solicitor review it — ground rent traps are common.
How Much Does a Lease Extension Cost in 2025? (Realistic Ranges)
For flats valued £200k–£600k:
| Lease Length | Typical Price Range |
|---|---|
| 130–150 years | £0 (no need) |
| 110+ years | £3,000–£7,000 (if ground rent is high) |
| 90–100 years | £6,000–£12,000 |
| 80–90 years | £8,000–£18,000 |
| 70–80 years | £15,000–£35,000 |
| Under 70 years | £25,000–£60,000+ |
Ground rent escalators dramatically impact cost.
Is a 110-Year Lease Long Enough?
Yes — in 99% of UK mortgage cases.
Most lenders only get nervous when leases drop below 90 years.
At 110 years, extending your lease is usually optional unless:
- Ground rent is aggressive
- Ground rent doubles every 10–25 years
- You’re selling soon and want to remove buyer hesitation
Can You Extend a Lease to 999 Years?
Yes — but only through an informal deal with your freeholder.
The statutory route gives you only +90 years.
If you want 999 years, your freeholder must agree privately.
But be careful:
Some freeholders offer 999 years but keep high ground rent, which reduces resale value.
How Accurate Are Lease Extension Calculators?
They are useful, but not precise.
Accuracy range: ±15–25%
Why they differ:
- Different relativity models
- Different assumptions about ground rent escalation
- Some calculators exclude marriage value calculations
- Some use outdated tribunal data
Use calculators for a baseline.
Use a surveyor for the final number.
Common Mistakes Homeowners Make (2025 Update)
1. Waiting until the lease drops below 80 years
This can add £10,000–£40,000 overnight.
2. Accepting an informal deal with hidden ground rent
Look for phrases like “review pattern” or “every 10 years”.
3. Using only one calculator
Always compare at least 3 tools.
4. Assuming online calculators include legal + surveyor fees
They don’t.
5. Not checking mortgage lender requirements
Some banks have stricter lease-length policies in 2025.
Checklist: What You Need Before Using a Lease Extension Calculator
- Your flat’s current market value
- The exact lease length in years & days
- Ground rent details (amount + increases)
- Service charge (optional but helpful)
- Whether you’ve owned the property for 2+ years
This ensures calculators don’t spit out inaccurate guesses.
Future Trends (2025 Reform Insight)
Leasehold reform changes are still being debated.
Two potential shifts:
- Marriage value may be abolished
This would make lease extensions significantly cheaper for leases under 80 years. - Standardised valuation formula
It could make online calculators more consistent and more accurate.
As of 2025, nothing has fully passed — but changes may arrive within 12–18 months.
FAQs
Q1: What is the average cost of extending a lease in the UK?
Most UK lease extensions cost between £6,000 and £30,000, depending on your lease length, property value, and ground rent. For flats with leases under 80 years, costs often rise to £15,000–£40,000+ due to marriage value. Using a lease extension calculator UK can help you estimate your premium before consulting a surveyor.
Q2: How are lease extension costs calculated?
Lease extension costs are calculated based on three main factors: ground rent loss, reversion value, and marriage value (if your lease is under 80 years). Calculators take inputs such as current lease length, property value, and ground rent pattern to estimate the cost. This gives homeowners a practical idea of what to budget for a statutory or informal lease extension.
Q3: Can I extend my lease to 999 years?
Yes, you can extend a lease to 999 years, but only through an informal agreement with your freeholder. The statutory route allows a 90-year extension with zero ground rent but does not provide a 999-year option. If you want a very long lease, using a lease extension calculator helps estimate costs and plan negotiations effectively.
Q4: Is a 110-year lease long enough?
Yes. Lenders in the UK are usually comfortable with leases above 90 years, so a 110-year lease is generally sufficient. Extending further is optional unless your ground rent escalates aggressively or you plan to sell soon. Calculating your potential premium using a lease extension cost calculator can confirm whether an extension is worthwhile.
Q5: Are online lease extension calculators accurate?
Online calculators provide useful ballpark figures, but results can vary by 10–25%. Variances occur because different calculators use different valuation tables, ground rent assumptions, and marriage value calculations. Always use them as a starting point, not a binding estimate, and verify with a qualified surveyor.
Q6: What happens if my lease drops below 80 years?
When a lease drops below 80 years, marriage value applies, causing costs to rise sharply — often by £10,000–£40,000. Using a lease extension calculator under 80 years can help you understand the potential premium before initiating the statutory extension process.
Q7: Do I need a surveyor after using a lease extension calculator?
Yes. Calculators only provide estimates. To negotiate with your freeholder or prepare for a tribunal, a qualified surveyor is essential. They can give a legally robust valuation, help challenge inflated quotes, and confirm the statutory premium accurately.
Conclusion
Lease extension calculators are incredibly helpful — but only if you understand what’s driving their numbers. When you mix calculator estimates with real-world factors like ground rent, marriage value, and statutory rules, you get a clearer, more confident view of your true cost.
Use calculators as a guide.
>Use a surveyor for accuracy.
>Use the 5-Box Framework™ to keep everything consistent.
And if you’re above 90 years?
You’ve got time — but don’t ignore ground rent or future buyer concerns.
Your next step: Before sending a Section 42 notice or negotiating, check the official government guidance (Gov.uk) to ensure compliance.
Related: Council Tax on Unoccupied Property in 2025: What Homeowners Should Know
