You can earn £7500/yr from a lodger without paying tax
You and dh would pay cgtax
your dc wouldn’t as he lives in the property ( PRR)
If you declare the property for cgtax each year you benefit from not paying cgtax on £3k of the gain each year
If you don’t declare each year and only when you sell you cannot accrue the yearly £3k benefit. You will only get one £3k allowance
You and dh combined will pay cgtax on 66% of the financial gain on the property ( assuming you own it with dc )
So
if each year it increases in value by £10k you and dh pay cgtax on £6,600 but you get £3k off that
so you and dh will be taxed on £3,600
Thats £1,800 each to be taxed
for higher earners it’s taxed at 24%
for lower earners it’s at 18% or
If you wait till you sell and it’s increased £10k / year and you sell after 2 years you and dh pay cgtax on £13,200 with £3k off that
so you and dh will be taxed on £10,200
That’s £5,100 each to be taxed
As an aside
Comparing those two taxable sums
£10,200 taxable sum declaring once
and
£3,600 each year for two years ie £7,200 taxable sums declaring each year
There are clear benefits to not leaving it till you sell