Without carrying out a full building heat loss analysis, and calculating its related energy consumption profile and the hot water requirements of the buildings occupants. You can not accurately design a ground source heat pump system.
The main problem with many renewable energy installations is that that compared to normal energy systems such as gas / oil or electric, the installed costs are generally much higher, so meaning the economies of scale are more limited. An oversized Heat pump will spend most of its time running under part load conditions, which can result in a shortening of the equipments lifespan and ultimately affect performance.
Under-sizing can result in a system that requires another heating system to be used, instead of the GSHP during periods of cold weather. This is known as an alternative bivalent system and is not very efficient. A top up system would be required to help the system meet its requirements. Whilst it is in fact fairly normal to have what is known as a parallel bivalent system, where two systems work together during periods of peak loads, the heat pump will work at maximum output providing the base load of the heating, while the other system tops up the temperature levels. It is vitally important to know the buildings and its occupant's energy requirements so that the most energy efficient and therefore cost effective system is designed, as generally the use of non renewable supplementary heating should not exceed 5% of the annual energy requirement.
It's not only the sizing of the heat pump that needs to be considered when designing the system. Different ground conditions will have an affect on the performance of the ground coil or borehole system used to collect heat from the ground. It is widely thought that ground source heat pump systems take their heat from geothermal heat however, this only occurs in a few regions of the UK.
The ground collections solar energy and almost all of the ground heating effect comes from the sun, even up to hundreds of meters below ground level. The type of ground ie, sand / gravel, rock, clay have different levels of heat extraction. Even the moisture levels of the ground will have an effect on the performance and therefore the design of the system.
Without a ground condition survey being carried out one could not even say whether or not a coil system is appropriate. If the ground conditions are such that there is let us say, 1m of topsoil over rock then the cost of the installation will vastly increase.