The move costs more than the move
Families model the big picture carefully — sale prices, ownership shares, monthly savings — and then get surprised by the first six months. The one-off costs of combining households routinely run to $15,000–$50,000 / £12,000–£40,000 before any construction, and because they land at the same time as the emotional upheaval of the move, they're the costs most likely to cause friction.
What families forget to budget
Selling and buying costs
If the plan involves selling one or two homes, the transaction costs are the biggest line: agent or estate agent fees, legal fees, property transfer taxes, and moving itself. Selling two homes to buy one can consume $30,000–$80,000 / £20,000–£60,000 in pure transaction costs depending on prices and jurisdiction — the two-homes-to-one guide works through the sequencing.
Duplicate everything, dispose of everything
Two households own two of most things. Disposal costs money (clearance, storage, skip or dumpster hire: $1,000–$5,000 / £800–£4,000), and the emotional cost of clearing a long-held family home is real — budget time for it, not just cash. Storage units meant to be temporary have a way of becoming a $200/£160 monthly standing charge for years.
Making the space work on day one
Even without a full conversion, most families spend on immediate adaptations: a second bathroom refresh, kitchenette appliances, new locks and doors, decorating the incoming generation's space so it feels theirs rather than borrowed. Typical range: $5,000–$25,000 / £4,000–£20,000. If a genuine conversion is planned, that's a separate and much larger number — see the conversion cost guide.
Professional fees
The paperwork that protects everyone costs money up front: legal advice on ownership structure, a co-ownership agreement, updated wills on both sides, possibly tax advice. Budget $2,000–$8,000 / £1,500–£6,000. It's the least skippable line on this page — skipping it is how families end up in the scenarios covered in death, divorce and separation.
Who pays for the one-offs?
Decide before the invoices arrive. Common approaches: split in proportion to ownership shares; the incoming generation pays for their own space's adaptations; or the one-offs come off the top of a house sale before proceeds are divided. Any of these work — what doesn't work is deciding afterwards.