The law relating to the extent of the Right of Easement to Light is now authoritively determined by the judgments delivered by Lord Chancellor Halsbury and Lords Macnaghten,Davcy, Robertson, and Lindleyinthe House of Lords appeal case - Colls v. Home & Colonial Stores Limited - which was finally decided on 2nd May 1904, and reported in Law Reports House of Lords Appeal Cases, pp. 179-213. In that case the facts (so far as it is necessary to set them forth in this place) were these.

The Home & Colonial Stores Limited were lessees of a building with ancient lights facing across Worship Street, Shoreditch. The building was used by them for business purposes. Colls, the defendant in the action and appellant in the House of Lords, proposed to build on the opposite side of the street. The building he proposed to erect was to be 42 feet high. The street was 41 feet broad at that place. The sills of the lowest windows in plaintiffs' place of business were 3 feet from the ground. Plaintiffs brought an action for injunction before the proposed building was erected, because they thought it would obstruct their light. The action was tried by Joyce, J., who found on the evidence that the proposed building would not materially interfere with the access of light to any of plaintiffs' windows, except two out of five windows on the ground floor facing Worship Street.

The room to which those windows belonged was used by plaintiffs as an office for clerks. It was 12 feet 10 inches high and 50 feet deep from the windows to the back, where there were no windows, so that it had been the practice to use electric light for the back part of the room even in the daytime.

The proposed building would not affect the value of the premises, and they would still be sufficiently lighted for all ordinary purposes of occupancy as a place of business.

Joyce, J., dismissed the action with costs. His decision was reversed by the Court of Appeal (Vaughan-Williams, Romer, and Cozens Hardy, L.JJ.), who granted an injunction to restrain the appellant from building so as to darken, injure, or obscure any of the respondents' ancient lights or windows, as they were enjoyed previously to the taking down of the old building at 44 Worship Street, with an order to pull down all the building which had been rebuilt so as to darken, etc.

This last part of the order was consequent upon the fact that, after Joyce, J., had decided in his favour and dismissed the action, defendant had gone on with the erection of his proposed building, and completed it before the decision of the Court of Appeal. On the facts found by Joyce, J., all the learned Lords concurred in finding that no cause of action was disclosed, and they upheld his original decision. The chief principles authoritively affirmed by the House of Lords in that case may be thus summed :-

(1) That the dominant owner is not necessarily entitled to claim a perpetual right to all the light which has reached his windows, so as to have a good ground of action if his neighbour intercept some of it.

(2) That to constitute a ground of action there must be such substantial deprivation of light as to make habitation uncomfortable in the case of a dwelling house, or in the case of business premises to prevent the plaintiffs from carrying on his business as beneficially as before.

(3) The test of the right of redress by action is whether the obstruction complained of is a nuisance.

Their Lordships had under consideration all previously reported cases which appeared to have any bearing upon the main questions laid before them for final decision in this case, and they have in effect affirmed that the principles of law now applicable are the same as before the Prescription Act was passed, and they have finally reaffirmed the principle of law enunciated by Lord Hardwicke more than seventy years ago, when he said: "It is not sufficient to say it will alter the plaintiffs' lights, . . . the law says it must .. be a nuisance."

Thus it may be taken that before any owner or occupier of premises for which the privileges of ancient lights are claimed by prescriptive right can hope to succeed in establishing a ground of action, he must be in a position to prove - (1) That he is entitled to assert dominant rights for his tenement over the land owned or occupied by the person whose action in building or otherwise obstructing his lights is complained of. (2)

That the action complained of really does cause a substantial diminution of the light which can reach the privileged windows. (3) That that substantial diminution of light is so great that the channels left free for light to reach the window will not be sufficient for the comfortable enjoyment of the premises if the new building complained of is erected or allowed to remain without alteration ; or, That the privileged premises can no longer be beneficially used as before in consequence of the action of defendant in building so as to obstruct part of the light to the windows.

It is practically impossible to lay down any exact hard or fast rules to bind a Court or Jury in deciding the issue about the sufficiency of the obstruction to constitute a ground of action in all cases of dispute.

A good general working rule which has been often applied and approved and accepted has been that if a clear 45 degrees of vertical angle is left unobstructed, the privilege of ancient lights is generally thought to be sufficiently safeguarded.

This means in practice that the height of the obstructing building must not exceed the horizontal distance between the two buildings by any more than the height of the sill of the lowest privileged window.

This common rule of practice is not, however, to be regarded as a binding rule of law.

In each case a number of circumstances may have to be taken into account to lead to a just decision, which shall secure to the owner or occupier of the privileged tenement all that he may reasonably be held entitled to continue to enjoy without imposing any greater burden on the servient tenement than the circumstances of the case strictly require.