Hi jonjo,
Draining the field may help to dry the ground out a bit but this probably will make little or no difference to any problems you may have with your house.
The fact that the field is wet may be due to many reasons such as
- the aspect of the field (i.e. is it low lying). If there is high ground surrounding it then all the rainfall may be naturally accumulating in the lowest point ie your field
- the outflow potential from the site may be poor. That is you may drain the field but unless there is the potential to get the drained water off the site (into a main drain/ stream or river) then your drains may be useless. You may need to clean out a main drain to achieve this which may mean the cooperation of your neighbouring landowners is required if the main drain runs through their land.
- the soil may have poor percolation to start with. In very heavy soils (with high clay content) the soil will act as a sponge for water and may not release this water that easily. It will be difficult to improve drainage in this case without spending a lot of money.
- the water table (level of ground water under the soil- you tap into this if you are digging a well) maybe high. Drainage may help here insome cases.
For the percolation test if your soil has poor percolation or the water table is high then this will be picked up when the trial hole is dug from looking at the soil itself (the soil will show this through colour changes & water lines)- I doubt if having it drained will have any great influence on the results.
For your foundations again the soil and its load bearing capacity will determine how deep you need to go to get solid ground for the founds.
If the proposed site of your house is wet it would be advisable to dig a drain around the proposed site boundary to try and minimise the amount of water coming onto the site (important at foundation stage and right throughout the build). Again the effectiveness of this drain will be influenced by the points above. There is no point pulling drains through the proposed site itself as you will only end up breaking them when you dig your foundations.
My advise would be to get a local site clearence expert/ diggerman out on site and get his opinion. Talk to neighbouring farmers and get the name of a drainage contractor and get him out on site for a second opinion.
The cost of drainage is a 'how long is a piece of string' question but I would reckon that taking into account the price of hiring a tracked digger+ operator (+ one other person (you??)) , land drainage pipe, drainage stones you would be talking about €500- €800 per acre. Then if you drain the whole field you will need to reseed it - further costs.
Hope this helps,
laragh