How much of our Savings should we use for 1st Time Self Build Mortgage?

My wife and I intend to build a house on her fathers land. We have been advised by the architect that a well designed high spec low energy A rated eco house would cost approx €400k (plus furnishing). This would be our 1st mortgage, we are currently renting, have 1 new born, have no loans, no other property, combined salary €104k, and have approx €150k of hard earned savings in the Irish Nationwide Flexi 21 earning 5%.

I had read that National Irish Bank have a good LTV mortgage offer. Looking at their website calculator, if we put all of our savings into the house and borrowed €250k that would be a LTV of 63%, term 25 years , monthly repayments would be €1501.80 , 5.41% APR. This sounds affordable.

Im not sure what to do with our savings the way the economy is right now. Should we be trying to put as much of our savings into the house as possible to get a good LTV rate? Or would it be more prudent in the current climate to invest our savings in equities which are relatively low and get a larger mortgage at at fixed rate? Any advice would be much appreciated.

400K With Furnishings seems very high.How big is the house your building as a friend has just finished 3000+ sqft house for 320k with A rating.Think another 25 k on furnishings all very high spec.Personaly I would keep some cash for that "rainy day"!If you search around you'll build the house a lot cheaper and then hold off on some of the cash.Say you build for 350k then use 100k and keep 50k as back up.Build on this money and then gradualy pay down your mortgage....But always have something as back up!!May not be as easy to get money off banks in future years!!