How to build a diversified portfolio comes down to one idea: own things that do not all fall for the same reason at the same time. That means spreading money across asset classes, within each asset class, and across countries and currencies, while keeping costs low enough that the benefit is not lost to fees.
The government-backed MoneyHelper service puts it simply: “spreading your money between the different types of asset classes helps lower the risk of your overall portfolio underperforming”, and “there’s no such thing as a ‘no-risk’ investment” (MoneyHelper). This guide explains the seven asset classes TID covers, what correlation means, how rebalancing works and where costs and home bias come in. It is education, not a model portfolio, and it does not suggest how much to hold in anything.
What diversification does, and what it cannot do
Diversification reduces your dependence on any single outcome: one company’s results, one landlord’s tenant, one country’s economy. If one holding fails, the damage is limited to its share of the whole.
It cannot remove market risk. When investors sell almost everything at once, most risky assets fall together, and a diversified portfolio still loses money. What it changes is the shape of the losses: fewer total wipe-outs, and a better chance that something is holding its value when you need to sell.
The seven asset classes
TID organises its coverage into seven desks. Strictly, funds are a way of owning the other six rather than an asset class of their own, but they are how most private investors diversify, so they get a desk.
| Asset class | Role it can play | Main risks |
|---|---|---|
| Cash and bonds | Spending money and an emergency buffer; income; gilts are free of capital gains tax | Inflation eroding real value; bond prices fall when interest rates rise; a borrower can default |
| Funds, trusts and ETFs | Instant spread across hundreds of holdings in one purchase | Charges; tracking a concentrated index; investment trust discounts |
| Shares | Long-term growth and dividends from owning businesses | Large, sudden falls; single-company failure |
| Property | Rental income; real assets | Illiquidity; borrowing; tax on rental income; concentration in one building |
| Gold and commodities | Behaves differently from shares and bonds at times; no reliance on a company or government paying you | No income; sharp price swings; storage and dealing costs |
| Private markets | Access to unlisted companies and assets, sometimes with tax reliefs | High risk; may be impossible to sell; you could lose everything invested |
| Alternatives | Infrastructure, digital assets, collectables and other assets outside the mainstream | Hard to value and to sell; costs can be high |
Two of those rows rest on official sources: gov.uk lists UK government gilts among assets free of capital gains tax (gov.uk), and the Bank of England’s Bank Rate stood at 3.75% on 9 October 2026 (Bank of England) while consumer prices rose 3.1% in the 12 months to August 2026 (ONS), a reminder that cash returns are measured against inflation. The rest describes how each asset works. Our desk guides go deeper, starting with bonds explained, property investment in the UK and how to invest in gold.
Correlation in plain English
Correlation measures how far two investments tend to move together. Statisticians score it from minus one (they always move in opposite directions) to plus one (they always move together). Zero means there is no consistent link.
Diversification works best with holdings whose correlation is low or negative. Two shares in the same industry, exposed to the same customers and costs, often rise and fall together; adding the second does little. A government bond and a share are driven by different things, so they do not necessarily move together.
Correlations are not fixed. They are measured from the past, and they can change, especially in a crisis, when assets that usually move apart can fall together. Treat a low historical correlation as a tendency, not a promise. Past performance is not a guide to future returns.
Diversifying within each asset class
Spreading across asset classes is only half the job. Ten shares in one sector are less diversified than they look, and a single buy-to-let is one building in one street. Funds solve much of this by holding many securities at once: a global equity fund can spread money across many companies and countries. Our guide to global tracker funds explains how they are built.
Check what an index actually contains. A fund that tracks a market dominated by a few very large companies is concentrated in them, however many names it holds.
Home bias
Home bias is the tendency to hold more of your own country’s assets than its share of global markets. For a UK investor it adds to exposures you may already have: a UK job, a UK home and perhaps a UK pension. Investing abroad spreads the economic risk but adds currency risk, because the value of overseas assets in pounds moves with exchange rates. Neither choice is free of risk; the point is to make it deliberately.
Rebalancing
Once you have chosen a mix, markets will move it. If shares rise faster than bonds, shares become a bigger slice than you planned and the portfolio becomes riskier than you intended. Rebalancing means trimming what has grown and adding to what has shrunk, to return to your chosen mix.
There are three common approaches: on a calendar date, such as once a year; when a holding drifts beyond a band you set in advance; or by directing new money to whatever is underweight, which avoids selling. Rebalancing has costs. Inside an ISA you pay no tax on income or capital gains (gov.uk). In a taxable account, gains above the £3,000 annual exempt amount (gov.uk) are taxed at 18% within the basic rate band and 24% above it (gov.uk), so selling to rebalance can create a tax bill.
Costs
Every layer of charges, from platform to fund to dealing, comes out of returns, and the effect compounds. A hypothetical illustration, not a forecast: £10,000 growing at 5% a year for 20 years becomes £26,533; at 4% a year, after a one percentage point annual cost, it becomes £21,911. The difference is £4,622.
More funds do not always mean more diversification. Several global funds may own the same large companies, so you pay several sets of charges for overlapping holdings. Look through to what each fund holds before adding another.
Liquidity and time
A diversified portfolio still needs a cash buffer, so that a bad month in markets never forces you to sell. Cash in a bank or building society is protected by the FSCS up to £120,000 per person per institution since 1 December 2025 (FSCS). Investments are protected up to £85,000 per person per firm if a regulated firm fails, but never against a fall in value (FSCS).
MoneyHelper suggests investing when your goal is more than five years away and you have a cash cushion of three to six months (MoneyHelper). Illiquid holdings, from property to private companies, need an even longer horizon. If you are starting out, our guide to how to start investing covers the steps before this one.
Questions to test your own mix
- If my largest holding halved, what would that do to the whole?
- Which of my holdings would fall for the same reason, such as rising interest rates or a weaker pound?
- How much of my wealth, including my home and job, depends on the UK?
- What do I pay in total charges each year, in pounds?
- Which holdings could I sell within a week if I had to?
- When did I last compare my mix with the one I chose, and what is my rule for rebalancing?
