OBR Sets Out How It Forecasts Migration Using Stay-Rate Model and ONS Population Data

The UK’s independent budget watchdog has published a new article explaining how it uses visa stay-rates and Office for National Statistics population projections to forecast net migration — figures that feed directly into government spending plans.

The Office for Budget Responsibility posted on social media this week directing readers to a newly published article on its migration forecasting methods, setting out how a “stay-rate” model — which estimates what proportion of visa holders remain in the UK after a given period — underpins its projections of net migration flows. The figures show that in its March 2024 forecast, the OBR expected net migration to average around 350,000 a year over the forecast period, up from around 290,000 a year in its previous November forecast.

Central to the OBR’s approach is its use of ONS National Population Projections (NPPs) as a key input for the size and age structure of the UK population. The ONS 2022-based projections assume long-term net international migration of plus 340,000 people a year for the UK from the year ending mid-2028 onwards — with around 858,000 people arriving and around 518,000 leaving annually under that principal scenario. The OBR supplements these with its own judgements about migration where necessary, and the two bodies now share stay-rate methodology, with ONS having adopted the approach for its 2024-based projections.

One distinction matters here: ONS projections are explicitly described as projections, not forecasts. They show what would happen if assumed trends continued — they don’t factor in future policy changes. The OBR, by contrast, produces genuine forecasts that incorporate economic conditions and policy decisions.

For Kent households and public services, the implications run across several areas. Kent County Council, NHS Kent and Medway Integrated Care Board, and local district councils all use ONS-based population projections to plan school places, GP capacity, housing and social care. Higher national net migration assumptions point to stronger projected population growth in parts of Kent — above all urban centres and areas near London — which in turn shapes infrastructure investment decisions. Local economic development bodies may also use OBR forecasts to assess future labour supply in sectors such as logistics, agriculture, care and hospitality, where migrant workers make up a notable share of the workforce.

Yet critics have long questioned whether official migration projections keep pace with rapid policy shifts — new visa rules, for instance, can alter flows before modelling catches up. The OBR itself has noted that historical errors in past population projections have often stemmed from mis-estimating future net migration. No Kent-specific figures appear in the OBR’s article; any county-level implications would require local-authority projections from ONS separately.

The OBR says the article is intended to improve transparency about how migration feeds into its economic and fiscal projections — the same projections the government uses when setting public spending envelopes for local government, health and education.