Hello Louise,
Sorry for the delay in getting back to you. Our team has looked at the ONS report you are referring to and our data. Here are some possible reasons for the difference you found:
1) In the ONS table notes, it is mentioned “Once a person either lives with a partner or has a child, they are considered to have formed their own family and are no longer counted as being part of their parents’ family even if they still live in the same household*. Therefore such people are deemed to be not living with their parents here.” -> which leads to the conclusion that LFS figures might underestimate the proportion of young people living with parents as, for example, lone parents (mostly single mothers), or multigenerational households would not be taken into account. So, see if after using the LFS definition of household and applying that to our data, the difference in proportions of 20-34 year olds living with their parents across the two surveys reduces.
*See page 40 of the attached document to see how households are defined in our study
2) Other things to take into consideration from the notes under the tables “Students living in halls of residence during term-time and living with their parents outside term-time are counted as not living with their parents here.” In UKHLS, some young people are enumerated in household grid (indall file) but could indeed live most of the year elsewhere (e.g. in the student halls).
3) There may be differential attrition with young people living with their parents less likely to attrit (when people move the risk of losing contact with them increases).
Hope this helps. If you have further questions or follow-up please let us know.
Best wishes,
Alita