Mr. Thomas's house is extremely valuable because it was designed and built by the man widely considered to be England's greatest architect, Christopher Wren. This is revealed to the gang by Trevor.
"Wren built that house, father says."
"The man who built St. Paul's."
Later, Trevor tells the gang he asked to see the house and the kindly Mr. Thomas showed him through it. The description of the interior beginning at this point and continuing throughout the story as the gang gradually destroys this irreplaceable dwelling provides verification that it really is--or was--designed and built by the great Sir Christopher Wren.
Trevor describes part of what he saw:
"It's got a staircase two hundred years old like a corkscrew. Nothing holds it up."
Since Wren died in 1723, the house would have to be well over two hundred years old--but the care and labor the boys put into wrecking it proves that it was built to last for much longer than that. Once the gang gets inside, the reader gets impressions of the elaborate craftsmanship that went into this old but lovingly preserved house.
Summers with hammer and chisel was ripping out the skirting-boards in the ground floor dining-room; he had already smashed the panels of the door. In the same room Joe was heaving up the parquet blocks, exposing the soft wood floor-boards over the cellar.
Such a house, if it had been preserved, would be an historical monument. People from all over the world would come to view it with wonder. What is especially horrifying about the destructors is that, as Greene emphasizes, these products of the most bloody and destructive war in human history assume they are creating their own work of art in thoughtfully demolishing an architectural masterpiece, smashing the stained glass windows, sawing through the bannisters, and finally arranging to have the ruined shell pulled down by the lorry when it leaves in the morning. The reader must be reminded of the works of painters like Jackson Pollack and other abstract expressionists.
The reader will also be reminded of the British schoolboys in William Golding's Lord of the Flies, who are marooned on a deserted island after their plane is shot down, and who are turning into savages.
Mr.Thomas' house is valuable because it is the only house whoch survived the bombs.