From the DCLG's English Housing Survey 2010-11:
3.1 Levels of overcrowding and under-occupation are measured using the ‘bedroom standard’ (see glossary*). This is defined by the difference between the number of bedrooms needed to avoid undesirable sharing (given the number, ages and relationships of the household members) and the number of bedrooms available to the household. A household is defined as under-occupied if it has at least two bedrooms more than needed, according to the bedroom standard. A household is defined as overcrowded if there are fewer bedrooms available than required by the bedroom standard.
3.1 Data from the three most recent years has been combined to produce the estimates discussed in this section of the report1. This is because the number of overcrowded households interviewed in each survey year is too small to enable reliable estimates to be produced for a single year.
3.2 The rate of overcrowding for 2010-11 was 3% of households. The rate for under-occupation, by contrast, was 37%.
3.3 In the last ten years, the rate of overcrowding has increased slightly, from 2.4% in 2001-02 to 3.0% in 2010-11. This rise was mainly related to an increase in levels in the social and private rented sectors, whilst the rate of overcrowding in the owner occupied sector remained unchanged over this period, Figure 3.1 and Annex Table 3.1.
3.4 Under-occupation was, overall, much more prevalent than overcrowding, and mainly concentrated in the owner occupied sector, where the rate was 49%, compared to 10% in the social rented sector and 17% in the private rented sector. The overall rate of under-occupation in England increased gradually in the last ten years, from 34% of households in 2001-02 to 37% in 2010-11. For owner occupiers the rate of under- occupation increased from 43% to 49%**. In both the social and private rented sectors there was a slight decrease in levels of under-occupation, Figure 3.2 and Annex Table 3.1.
* From the glossary: "The ‘Bedroom Standard’ is used as an indicator of occupation density. A standard number of bedrooms is calculated for each household in accordance with its age/sex/marital status composition and the relationship of the members to one another. A separate bedroom is allowed for each married or cohabiting couple, any other person aged 21 or over, each pair of adolescents aged 10-20 of the same sex, and each pair of children under 10. Any unpaired person aged 10-20 is notionally paired, if possible, with a child under 10 of the same sex, or, if that is not possible, he or she is counted as requiring a separate bedroom, as is any unpaired child under 10.
This notional standard number of bedrooms is then compared with the actual number of bedrooms (including bed-sitters) available for the sole use of the household, and differences are tabulated. Bedrooms converted to other uses are not counted as available unless they have been denoted as bedrooms by the respondents; bedrooms not actually in use are counted unless uninhabitable."
** Hardly surprising, really. If people stay living in the same house after their children have left home, and then still stay living in the same home once they are widowed, under-occupation among owner-occupiers will tend to increase over time. It is only when a home is sold or re-let that the new occupants choose something 'just big enough'.
Conversely, we would expect the level of overcrowding to increase over time as more and more people (those who didn't get on the ladder in time) have to share whatever housing is left over (new supply being strictly limited).