There is only one definitive authority when it comes to marking boundaries and that is the UK Land Registry.  Your property boundary is marked on your title deed and that is the only document which counts when it comes to locating and defining any property boundary.

Depending on the age of your property and when the last land transaction took place, your boundary will be marked by a red line on the illustration which accompanies your title deed.  If you are a renting or a leasing tenant of a property, that document will be in the possession of your landlord or their agent, but it will also be available for purchase or viewing from the land register.  If you are the home or business property owner, then either you or your solicitor should have a copy of your title deed, but if you don't have it, it will always be on file, available from the land registry.

In relatively old properties, there are still some rare cases where a title deed drawing does not accompany the title deed, and the land and buildings contained in the deed are only described in words, with measurements from fixed points described in the deed to facilitate accurate delineation by a chartered surveyor or land agent, but in my experience, these are somewhat rare now, but they still exist.  

When we find one of these properties, there is one trick we use to get to the truth, and often it works very well.  We simply draw down the titles of all of the neighbouring properties and hope that their title deeds are more modern and up-to-date, with drawings and red lines.  When we can see exactly where their boundaries are on the map, it's usually fairly straightforward to "subtract" our old title boundary from what is left, if you see what I mean.  It's a little bit of detective work, but if you have ever read a very old title deed and try to make sense of the somewhat vague, (some say deliberately vague) descriptions of where the boundaries lie, then this method can save you a whole lot of headaches.  I would say this; it's not completely fool proof, you can still be left with questions and if you're not sure about it, there only one option and that's to call a surveyor.

What is a surveyor going to do?

A chartered surveyor is the authority in anything to do with boundaries.  They are trained and qualified in the field of land and property, and their experience is invaluable when it comes to establishing the exact location of a boundary which is not obvious on a title deed.  I would add that even on some newer title deeds, the accompanying title drawing can be so small or ambiguous, that plotting the exact location of the boundary line and corner points is still difficult, so if you are having difficulties or maybe even a dispute about it with your neighbour or neighbours, then a surveyor can be your answer.  What I normally suggest at this point is that each party agrees to select a surveyor who is unknown to both or all parties.  No-one wants to be seen as having a "surveyor friend" who will act in favour of one party or the other, so neutrality is important.  Depending on the scale of the problem, the cost of a surveyor carrying out the survey and visiting the property to insert plot markers will cost anywhere from a few hundred pounds to over £1'000, so be aware of that.

I have known architects to think they know best on this topic and proceed without the advice of a surveyor, and get it completely wrong.  Whole boundaries of fencing erected in the wrong place has to come down and be rebuilt on the true boundary at huge expense, which has to be met by the architect or house builder who instructed the fencing contractors incorrectly.  Suddenly the cost of a surveyor seems like money well spent.

I have also attended old ex-council properties to replace fencing where the original council builders just built wherever they felt the fences should go, and sometimes they have been wildly off the mark, by several feet.  When those properties were sold off, that typically generated a fresh title deed with a red line drawing showing the boundary, but no-one thought to check it until the old fencing needed to be replaced and suddenly you discover that the garden ground is either much bigger, or much smaller than you thought.  That can kick-up all sorts of issues with whichever firm of solicitors conveyed the property, and of course, with the local council, but the one rule comes into force; whatever the title deed says is gospel.

So there you have it.  Check your title deed if you want to know exactly, definitively and legally, where your boundary line is.  From my own experience, I would hazard a guess that around 15-20% of all property fences are not quite where they should be, and for all sorts of reasons.  The original fencer might have been having a lazy day, or left the post positions to the apprentice.  Corners get cut. Someone measures in feet when it should have been in metres. (Yes, they do lol)  Obstacles like large mature trees or drains and manhole covers are in the way, so people just go around them, and the fence becomes a new false boundary over time. People don't want to dig out the old concrete from the old rotten fence posts, so they just cut the fence posts off at ground level and erect the new fence on one side or the other, and they keep doing that every 10-15 years or so until the fence is 3 or 4 feet from the original boundary line.  This is how it happens, so check your title deeds and see where the new fence should actually be before you proceed.  To me it's important, and in some cases it made the difference between having room to build a driveway for vehicle access or not, and that one factor can add significant extra value to any property. 

Here is where you can find your title deed in the UK.

For property in England and Wales, start here; https://www.gov.uk/search-property-information-land-registry

For property in Scotland, start here; https://www.ros.gov.uk/

For property in Northern Ireland, start here; https://www.nidirect.gov.uk/articles/searching-land-registry