Tax planning - potential land sale

Advice / thoughts needed. Situation is as follows:

- my elderly parents are in the rather fortunate situation that they own a piece of land ca 1.5 acres adjacent to their house. Some of it is steeply sloping but we think it should be possible to develop at least 2 * 4 bed houses on there, each of which would have at least half an acre of land.

- parents have already bought another house and are in the process of moving out, after which their existing house (also likely to be suitable for knocking down and redeveloping) will be put on the market.

- if they sell the land with planning permission, we estimate it will be worth circa £500k and would, I believe, be subject to CGT.

- my fear is that if they do this, the net amount after CGT will also form part of their estate at some point and thus be subject to IHT as well (I don’t know exact figures but with shares and the money from the sale of their current home I think they’ll reach the IHT threshold). I would love to think that they *could* themselves sell the land, gift away the sale proceeds and achieve relief from IHT via the 7 year rule but they are both having health issues so I don’t believe this is a wise route to take.

What I was wondering is whether myself and siblings could buy the land at current fair market value (without planning permission), which would mean that my parents would still crystallise a small gain, but relatively limited. As siblings we would then apply for planning permission and, if successful, market the land. We would then have to pay CGT but this would be spread across three people and (hopefully) not captured for IHT any time within the next few years.

My thinking is that, if acceptable to all parties, this would avoid the risk of one lot of sale value being subjected to both CGT and then IHT.

Thoughts welcome.