The company is claiming that Finers acted negligently on a 1997 property transfer and so breached its contract and duty in tort.
Neonhall is a private company owned by Colin Tett, which last year took over formerly listed company UK Land, of which Tett was the chairman.
In 1996 Finers advised Neonhall on the acquisition of a 13-acre estate in Sussex, which included a number of internal access roads. Neonhall then divided up the estate with the intention of selling off the plots.
In 1997 Finers advised on the sale of three plots and a portion of the access road to a company called Saxon. However, the section of the road that was sold was also necessary for access to a further plot (plot 11), which now remains unsold.
The claim says that Finers did not reserve any express right of way over the internal road or change the standard form of transfer, initially drafted in 1996. As a consequence, the subsequent value of plot 11 has been reduced by £250,000 because there is no longer a satisfactory right of way, Neonhall claims.
It is understood that the Finers solicitor that drafted the document is no longer with the firm.
Finers Stephens Innocent managing partner Anthony Barling said: “The matter's now in the hands of our lawyers and we're putting in a defence.” As yet no date has been set for a hearing.