Household wealth is profoundly important for living standards. We show that wealth inequality in the UK is high and has increased slightly over the past decade as financial asset prices increased in the wake of the financial crisis. But data deficiencies are a major barrier in understanding the true distribution, composition and size of household wealth. We find that the most comprehensive survey of household wealth in the UK does a good job of capturing the vast majority of the wealth distribution, but that nearly £800 billion of wealth held by the very wealthiest UK households is missing. We also find tentative evidence to suggest that survey measures of high-wealth families undervalue their assets – our central estimate of the true value of wealth held by households in the UK is 5% higher than the survey data suggests.