Most people building a New House prefer to go for the 'Slinkies' laid in a trench, or a 'radiator', because it is so much cheaper to instal.
For those with shallow trenches, it can be cost free. I know someone who has enough land for the slinkies, bought a second hand JCB on EBay, dug his own trenches to lay the slinkies, filled them back in, and resale value of the JCB on EBay is still worth what he paid for it.
Radiators (aka Compact Collectors) are a mesh of plastic pipes that are usually hung vertically in a deep narrow trench, or laid flat in a wide trench. Both of these are getting warmth from the current season's solar gain, and also the rainwater that falls on the ground they are in,. The ground temperature fluctuates seasonally, but there is enough mass to get through the winter. The required number of collectors is dependent on the predicted heating load of the house.
If you have enough land, you would NOT plant boreholes as they are fearfully expensive and require a professional drilling crew. But their benefit is that they get heat from Deep Down.
In our case we had to have boreholes, as the garden is too small for 'slinkies' and too narrow for a deep trench with a 'radiator' array of pipes - it would undermine the house foundations, and the neighbour's hedge.
So £5,000 poorer, we do at least have the best solution, because the soil 48metres down is dense and undisturbed and there's a lot of it. The boreholes will last hundreds of years, rugged pipes set into Bentonite. You could replace the heatpump, or even rebuild the house a few times, using the same holes!
The 'cylinders' of clay reach down 48m deep into the earth, and there is nowhere else for the heat we dump into it to go to than to stay there. And much of it will be returned when we withdraw the heat in the winter. It is like charging a battery. Even in Winter, there are sunny days and the solar panels will work. In our case, the 'store' is uninsulated, the distance sideways extends to near infinity.
Here's an idea for New Builders: The solar charging pipes could be laid flat (horizontally) in an insulated stack of bricks (second hand or waste) or GGBS concrete, Blocks of Paraffin wax, or even quantities of re-used or contaminated soil, all set in a very big 'box' of polystyrene foam. If you dig the hole for the 'box' you could refill with the earth you took out. Then you can run a radiator of Solar Charging pipes through this 'store' - the same pipe putting heat in as taking it out. And Build the House above!
But how big do you make this store for a whole season? One metre deep? Two metres? Four metres? Perhaps insulate the top and sides, but not the bottom, so that it extends vertically downwards to no limit, but does not lose heat upwards or outwards. This could be an expensive question to answer by trial and error, although it could be computer simulated.
• Cheapest and most sustainable solution would be to re-use the earth you took out to dig the box - earth neutral, and just requires a digger. You can even bury other people's soil if they have a surplus.
• Most thermally effective solution would be a large quantity of blocks of paraffin wax - they remain the same temperature, but go on absorbing heat, and then releasing it almost for ever - a Phase Change Material.