For many, moving in with a partner is a major romantic milestone. It often leads to a practical question: “What do we do with my flat?” If you decide to keep your original home and rent it out, you have officially joined the ranks of the “Accidental Landlord.”
While it seems like a straightforward way to cover your mortgage or build an investment, renting out your former residence triggers a series of tax obligations that many people overlook until they receive a “nudge letter” from HMRC. At Felix Accountants, we see hundreds of couples who didn’t realize that a simple change in living arrangements could lead to complex tax filings and potential penalties.
Here is everything you need to know about the tax implications of renting your first home when you move in with a partner.
1. Income Tax: Your New “Second Job”
The moment a tenant pays you rent, you have started a business in the eyes of HMRC.
The £1,000 Property Allowance
If your total rental income is less than £1,000 per year, you generally don’t need to do anything. However, for most landlords renting out a whole property, income will far exceed this.
Registering for Self Assessment
If your rental income is over £1,000, you must register for Self Assessment. You must notify HMRC by 5 October following the tax year in which you started receiving rent.
-
Example: If you moved in with your partner and started renting your flat in September 2025, you must register by 5 October 2026.
How Much Tax Will You Pay?
Rental profit is added to your other income (like your salary).
-
Basic Rate (20%): Total income between £12,571 and £50,270.
-
Higher Rate (40%): Total income between £50,271 and £125,140.
-
Additional Rate (45%): Total income over £125,140.
2. The Mortgage Interest Trap (Section 24)
Ten years ago, landlords could deduct their full mortgage interest from their rental income before paying tax. This is no longer the case.
You now pay tax on the full amount of rent minus “allowable expenses” (like insurance and repairs). You then receive a 20% tax credit for your mortgage interest.
-
The Risk: If you are a higher-rate taxpayer (40%), you are still paying 40% tax on the income used to pay the mortgage but only getting 20% back in relief. This can lead to situations where your “cash flow” is positive, but you are actually losing money after tax.
3. Stamp Duty (SDLT): The Cost of the “Next” Home
If you move in with your partner but decide to buy a new home together while keeping your old one, you will likely hit the Stamp Duty Surcharge.
In 2026, if you own one property (your original home) and buy another, the new purchase is considered an “additional dwelling.” This triggers a 5% surcharge on top of the standard Stamp Duty rates. On a £400,000 house, this surcharge alone adds £20,000 to your moving costs.
The 36-Month Refund: If you sell your original home within 36 months of buying the new one, you can usually claim a refund of that 5% surcharge.
4. Capital Gains Tax (CGT): Losing Your Relief
While you live in your home, it is exempt from Capital Gains Tax thanks to Private Residence Relief (PRR). However, the moment you move out and rent it, that exemption starts to “tarnish.”
When you eventually sell the property:
-
You get relief for the years you lived there as your main home.
-
You get relief for the final 9 months of ownership, even if you weren’t living there.
-
The remaining period (the rental years) is taxable.
The Rate: In 2026, CGT on residential property is 18% for basic rate taxpayers and 24% for higher rate taxpayers.
5. Don’t Forget the “Consent to Let”
Technically not a tax, but a legal must: You must notify your mortgage lender. Renting out a property on a standard residential mortgage without Consent to Let is a breach of contract. Lenders may increase your interest rate or demand immediate repayment if they find out via HMRC data sharing.
6. How the Let Property Campaign Can Help
If you moved in with a partner years ago and haven’t declared the rent, the Let Property Campaign (LPC) is your best solution. It allows “Accidental Landlords” to come forward voluntarily.
-
Lower Penalties: Because the mistake was likely an oversight (Careless) rather than a deliberate attempt to cheat, we can often negotiate 0% or very low penalties.
-
Catching Up: We can help you file for multiple years at once, ensuring you are fully compliant before you buy your next home together.
Frequently Asked Questions (FAQs)
Q1: My partner and I aren’t married. How does that affect the tax?
If the property is in your name only, the income is 100% yours for tax purposes. If you own it jointly, the income is usually split 50/50. Being unmarried means you can’t use “Form 17” to shift income to the lower-earner as easily as married couples can.
Q2: Can I deduct the cost of the new furniture I bought for the tenants?
No. You cannot deduct the initial cost of furniture. However, you can claim Replacement of Domestic Items Relief when you eventually replace those items (like a broken sofa or fridge).
Q3: What if my rental income doesn’t cover my mortgage?
You may still owe tax. Because you can’t deduct the full mortgage payment (only the interest, and only as a 20% credit), you can have a “taxable profit” even if your bank account shows a loss each month.
Q4: Does HMRC really know if I’m renting out my old flat?
Yes. HMRC’s “Connect” system tracks Land Registry changes and matches them against the electoral roll and Tenancy Deposit Schemes. If you are registered to vote at your partner’s house but still own your old flat, a “nudge letter” is often inevitable.
Q5: I only plan to rent it for a year. Do I still need to tell HMRC?
Yes. There is no minimum time limit. If you earn over £1,000 in a tax year, it must be reported.
Don’t Let Your “New Start” Be Ruined by Old Tax
Moving in together should be an exciting time, not a source of future legal stress. If you’ve recently become an accidental landlord, let Felix Accountants review your numbers and handle the HMRC registration for you.