An extension of a certain size could have been built under what are known as permitted development rights which are allowed by a Statutory Instrument titled "The Town and Country Planning (General Permitted Development) Order"
Unless that extension followed the requirement set out in the GPDO, it should have been granted planning permission by the local planning authority. 4 years after being erected, even if planning permission was not granted the building became lawful, because the council only had 4 years to issue an enforcement notice objecting to the extension being built.
All building work should also have had building control approval and it would be the building control department of the council that may know when this was completed, because that department would have issued a completion certificate , if they knew about the building.
That is always the problem. Did the builder tell anyone in authority about the extension?
The point is that this will not really effect he situation you now find yourself in.
The phrase "common yard" can mean many things, although it would normally be able to establish what was the use allowed to the non-owners of that yard. Property with legal rights over another person's property is known as the dominant land. Your land would be known as the servient land, which is there to "serve" these other properties.
Your quote from No. 3's deed clarified one such right.
"A right of way over such part of the common yard hereinbefore referred to as may be necessary for the purpose of access to and from the rear of the property hereby conveyed and the back road ten feet wide hereinafter referred to"
That explained that a right of way was granted, although the words "the common yard hereinbefore referred to"
imply that in the original deed for No. 3 there was a reference to what the common yard could be used for, in addition to providing a right of way to and from the back road.
That is why older deeds may have to be looked at to fully understand what rights the other properties have over the common yard.
If by erecting a fence that has a gate installed in it, you do not substantially obstruct a property owner from enjoying any right to use the common yard that were set out in a deed, then the fence cannot be the cause of an action for trespass. The cause of action has to prove a "substantial obstruction " to any granted right over your land.
If this is now only capable of being a right to hang washing, then the fence you want to erect is not likely to prevent that from happening as long as the dominant land owner arranges for a washing line to be fixed between two buildings or two poles. You don't have to provide any line, although a dominant land0owner can arrange to have a line positioned over the common yard at his own expense. That would be considered as an ancillary right along with the right to actually hang washing over the yard.
For a neighbouring property-owner to claim that you cannot erect a fence, it would first of all be necessary for him to provide you with deeds that set out exactly what rights over your land were granted to that property.
That would then allow an analysis of whether a fence with a gated access was going to cause a substantial obstruction to those rights.
A man just saying "you can't do that" will never be enough. That man needs to show why you cannot do that with written deeds setting out his rights so that you can ascertain whether those rights will be interfered with substantially if a fence was erected..
Why a man who rents out his property to tenants, wants to interfere with your use of your own land, even though it is burdened by certain easements over it, confuses me. That man will need to prove to a court that this fence substantially obstructs the right of his tenants to hang out their washing even though no one has ever one so.
The phrase "bullshit baffles brains" seems to be his method of applying pressure.
Saying "I am right" without providing any proof of why he is right is what is apparently happening.
You need to understand that.