I rent out my spare room — how does the rent-a-room scheme work, and do I need to tell HMRC?

At £450 a month you are on about £5,400 a year, so yes — you can ignore it completely for tax purposes. Under the rent-a-room scheme the first £7,500 of gross rental income per tax year from letting furnished accommodation in your main home is tax-free, and the relief applies automatically if you are under the threshold: no registration, no forms and no need to tell HMRC. If the income were shared with someone else, the £7,500 limit is halved to £3,750 each. If your receipts ever rise above £7,500, you would need to register for Self Assessment and report it — either by taking the flat £7,500 allowance (simple, no expense claims) or by declaring gross rent and deducting actual allowable expenses, whichever gives the lower taxable profit. Even under the threshold, keep a simple record of the rent received so you can show you were within the limit if HMRC ever asks. The scheme only covers your own home; it does not apply to buy-to-let or separate properties.