‘Cost only’ sale - any tax implications?

Can I build a home and sell it to my son at cost only?

A transfer between 'connected persons' (and father and son are deemed connected persons) is treated, for capital gains tax (CGT) purposes, as a transfer at present market value. See www.hmrc.gov.uk/manuals/cgmanual/CG14530.htm. This means ‘the price which that asset might reasonably be expected to fetch on a sale in the open market’. So as far as CGT is concerned, it won't help to sell it to your son at cost price only. But stamp duty land tax (SDLT) (usually) only looks at what is actually paid, so it will help for SDLT.