Hello I am a solicitor with over 15 years experience in practice and I will help you with this.
In order to help you I need more information. Do you own the house? Do you know if he owns his and if not do you have the same landlord?
Also where is the fence now and what is the story fo the concrete posts in your property?. (ie why are they there and hat is/was ther purpose?)
The following answer assumes you are both owner occupiers.
In order to work out the precise boundaries your first step should be the information provided by your conveyancing solicitor. There should have been a report on property or similar document prepared by your solicitor at the time of purchase. This should set out the extent of the property and the boundaries and also the reason for the solicitor coming to this conclusion. If you no longer have this document then it may be that your old solicitor has (although they are only required to keep documents for 6 years).
You can also use this link to go to the land registry (https://www.gov.uk/search-property-information-land-registry) where you can download a copy of the title plan. This will show the general boundaries of the property. The price for this is £3.
If the properties are garage linked then I am guessing that they are part of a development that was all built at the same time. What is the position with other houses nearby? Do their boundary fences follow the line you suggest? If so it would suggest that that the boundary may lie in the same place in your garden too. You can also obtain copies of the title plan for nearby properties.
Is your neighbour responsible for the fence or are you?
Once you have armed yourself with this information you should talk to you neighbour about where the boundary lies and therefore where the fence should go. Hopefully your neighbour will agree with you. Remember this must all be done politely and with tact. You are just trying to make sure it is all done properly (It may help to point out that if it is put in the wrong place then he has a potential problem with any future occupiers of your home who may discover it when purchasing) and are pleased that he is fixing the fence. Issues like this if handled badly can poison relations between neighbours for years to come.
If you can not reach agreement than you will need to consider instructing a solicitor or contacting Royal Institute of Chartered Surveyors at www.rics.org or on 024 7686 8555. They run a scheme where they can put you in touch with a surveyor who will give you 30 minutes free advice. You may also want to consider using a mediator to help resolve the dispute try this link for mediators. (http://www.resolution.org.uk/mediation/).
Remember these things are best done by agreement if possible.
I hope this answers your question but please do not hesitate to seek clarification.
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