Dealing with damp

I would really appreciate some opinions/advice on the following.
I live in an upside down house,bedrooms down stairs,as the house is built on a gentle slope. The house is currently being renovated and at the moment has no rainwater goods etc. On friday we had heavy sustained rain and on saturday when I went into a bedroom that is only slightly below ground level at one point(only bedroom below ground level and only room that was affected with dampness), I noticed that the floor looked quite wet. I lifted the floor tiles some were very wet underneath. Prior to the refurb we lived in the house and the room in question was actually our bedroom. We never noticed any damp, the carpet was not rotten etc.

A couple of questions,

As there are no gutters etc to take the water to drains at the moment would it be possible that the water would have seeped through the soil and found its way down to the bedroom. If this was the case it would suggest to me that the damp proof membrane which is possibly 40 years old has broken down.

I am planning on digging down the side of the house to see how far down an external memebrane goes. I was also thinking of painting the room, walls and floor with a liquid DPM, has anyone used these and of so are they any good? Would this just push the water somewhere else, ie up the walls

Any thoughts/comments very gratefully received

Thanks