The judges gave guidance on the interpretation of the phrase 'within the curtilage' as used in part 1 of the Planning (Listed Buidlings and Conservation Areas) Act 1990.
Skerrits of Nottingham Ltd, tenants of the Grade 2* listed Grimsdyke Hotel at Harrow Weald had appealed to the secretary of state for the environment against a listed building enforcement notice issued by Harrow LBC in September 1996.
The secretary of state dismissed the appeal on May 26 1998, but that decision was overturned by deputy high court judge George Bartlett QC on March 22 last year.
Lord Justice Robert Walker told the appeal court the contravention alleged in the enforcement notice was 'the removal of existing timber-framed windows and the installation of white plastic double-glazed windows' in a stable block within the hotel's grounds without listed building consent.
The notice required removal of 19 unauthorised windows, and reinstatement with the old type of window, within six months.
The main building was designed as an English country house by Richard Norman Shaw, the eminent Victorian architect. It was for many years the home of W.S. Gilbert, and later his widow. It has not been used as a private house since 1937 and its use as a hotel began in 1975.
The judge said the appeal turned on whether the stable block could be said to be within the 'curtilage' of the listed building within the meaning of part 1 of the 1990 Act.
Section 7 of the Act contains a general prohibition on the unauthorised carrying out of any works to a listed builty which would affect its character as a building of special architectural or historic interest.
Skerrits of Nottingham argued successfully before deputy judge Bartlett that the secretary of state had erred in law by 'overlooking the principle (if there is such a principle) that the curtilage of a listed building is confined to a small area around the building,' said lord justice Robert Walker.
But, overturning the decision and upholding the enforcement notice, the judge said: 'I respectfully doubt whether the expression 'curtilage' can usefully be called a term of art.
'That phrase describes an expression which is used by persons skilled in some particular profession, art or science, and which the practitioners clearly understand even if the uninitiated do not.
'This case demonstrates that not even lawyers can have a precise idea of what 'curtilage' means. It is a question of fact and degree.
'In the context of what is now part 1 of the Act, the curtilage of a substantial listed building is likely to extend to what are or have been, in terms of ownership and function, ancillary buildings.
'Of course, physical layout comes into the matter as well. In the nature of things the curtilage whithin which a mansion's satellite buildings are found is bound to be relatively limited. But the concept of smallness is in this context so completely relative to be almost meaningless, and unhelpful as a criterion.
'It follows that in my judgement the inspector and the secretary of state did not err in law in making no reference to smallness. I would allow this appeal.'
Mr Justice Alliott and Lord Justice Henry agreed.