A 'HIGHLY probable' fall in predicted household size is likely to force council bosses to plan to build more homes, rather than less, in the next 20 years.
With the number of old people, who are more likely to live on their own, increasing, there will be more pressure on homes in the future.
And the greater the tendency for people to form separate, smaller households, the higher the number of homes that will need to be built to accommodate them.
Cheltenham and Tewkesbury Borough councils and Gloucester City Council are all working together to try and figure out where houses should be built and how many up until 2031.
And in a bid to make sure they get their numbers right Cheltenham commissioned an independent review of household formation rates.
The report, compiled by the Cambridge Centre for Housing and Planning Research (CCHPR), was presented at a meeting of the council's overview and scrutiny committee on Wednesday evening.
Neil McDonald, from the organisation undertook the report, and he told the meeting via loudspeaker telephone: "The key point to make is that there are three things that drive average household size: The tendency of people to set up households, the ageing effect of older people tending to live in smaller households and marital status.
"Over the next 20 years because of the existing age profile we are likely to see a much faster growth in the older age group than we have seen in the last 10 years.
"As a result the effect of driving down average household size will actually be much stronger.
"It seems to be highly probable that there will be a fall in household size."
As such a Cheltenham Borough Council working group, set up to examine these issues, has recommended that the councils continue to use the same government household formation figures which they have been already when planning for future demand.
Councillor Tim Harman (C, Park), who chairs the working group, said they had found the household figures which have been used up to this point in the joint core strategy process to be 'valid'.
He said the independent review was 'important' in making sure the councils' plan is built on a solid evidence base.
He said: "I don't think that anybody wants to see a free-for-all from developers. This is about developing a sound strategy."
Based on the evidence presented by CCHPR, the working group suggested it would be prudent for the councils to continue to plan on the basis of the original household formation rate figures.
The working group also said the authorities should consider building flexibility into the strategy to take into account new information if and when it comes to light.