Should you choose indeed have this then it is better practise to dig it away from the wall structure thus getting rid of the risk of water. Some properties have only slightly high ground levels. You may simply need to make a call on this but remember that it might be a risk if the levels are not lowered.
Now that your ground levels are lower than your internal floors, why don’t look at other sources of moisture ingress which may be affecting the property prior to choosing your damp proof membrane layer. Here is a set of external resources that need to be examined.
Are your gutters overflowing and allowing water a direct way down your walls, leading to staining? If you are then clean them out! The humidity that has run down the walls may reveal in salting or staining internally. This is alright, you can deal with that with a Penetrating Damp Surveys proof membrane.
All properties built within the last 50 years, at least in the UK, needs to have a physical barrier inserted in to the brick/block work at 150mm above external ground levels. These are generally commonly described as a damp proof course (dpc). This form of damp proof membrane put into the mortar course is a highly effective barrier to deal with dampness from the floor moving up through capillary action to manifest inside within your property. Therefore first it is very important to establish whether you have one.
That might not be considered a physical barrier since there are also chemical damp proof walls which are being used on properties which weren’t built with a physical dpc or have been retrofitted to the structure if the original was defective. Evidence of this can be seen in small holes which have been drilled into the mortar course and later filled in, 150mm above ground level. This might have been done to deal with damp. These types of damp resistant membranes are now utilising better technology from that used twenty years previously. Take note these kind of products will still need skilled application and will require re-rendering up to 1m internally with a sodium retardant additive. The object rendering process will take time and will be presenting wet trades so you will be unable to redecorate quickly – maybe up to 6 several weeks afterward. It is important to note that if your property has no uniform mortar course than the application of a chemical damp proof membrane will be extremely difficult. Be wary if someone offers you this damp evidence membrane option.
Paint applied damp proof membranes are also an option. This specific is not just a matter of slapping a lttle bit of black bitumen paint on the wall and hoping for the best. There are plenty of “miracle” cure, paint-on options. Surprisingly enough these can actually cause more problems than are there in the beginning. Preparation is the key when applying this kind of product and will be the in it working, not working or so that it is even worse. So be very clear to get this right and understand the amount of time necessary to get the prep right. The most frequent problem with this type of application is the product. If it really does deal with damp, it may form a barrier and either move damp to where you haven’t treated or push damp higher in the wall. These products also will not get very sympathetic to listed buildings and you might not be able to ever get them off!