The name Debenham has its origins in a Suffolk village, a hamlet on the River Deben about 12 miles north of Ipswich. The name of the river is derived from the Anglo-Saxon for "deep valley", but just where this valley might be is not clear. The village web-site is most informative, and there is also a family history site dedicated to other names associated with the village.
A great deal of research into the origins of the Debenham family and its several lines of descent has been carried out by Professor Frank Debenham, O.B.E., (1883—1965), and has been published in his privately printed book, "Seven Centuries of Debenhams" dated May 1957. There is more about Frank Debenham in the section about Debenhams who have made their mark in the world. The following summary owes much to his work.
The earliest reference to the name appears to be a Lucas de Debenham, a man of high estate, probably Sir Lucas, who evidently lived in the area of Debenham in AD 1165. At that time the designation "de Debenham" only indicated that the bearer came from that place, as surnames were not in general use. About two hundred years later the prefix "de" died out, and the name became a surname as we know it today.
In the 14th and 15th Centuries there was a line of Gilbert Debenhams (five of them!) who lived at Wenham Hall in Suffolk. Their Coat of Arms appears at the entrance to this web site and has been used to illustrate much written work on the family, but its entitlement is not now known. Of these Gilberts, several represented Suffolk at the parliaments that were held at that time, one was Sheriff of the county in 1395, and one was knighted in person by Henry VI at Leicester in 1426. There is more about Wenham Hall at the CastleUK.net site.
The lines of descent following the various Gilberts are obscure, and likely to remain so. However, three areas are now apparent as centres of population for Debenhams of the 16th Century and later. One of these is Sapiston, a few miles north-east of Bury St Edmunds, another is centred on Redgrave, some 12 miles to the east and including the villages of Botesdale and Rickinghall, while the third is south of Bury and includes the villages of Thorpe Morieux, Alpheton and the Bradfield group.
Of these, the Debenhams descended from the Bradfield area include the founders of the Debenhams stores group. This line is now represented by a Baronetcy based in Dorset, but a parallel line of the same descent became farmers and agricultural workers in Suffolk. The Sapiston line has been researched in depth by Frank Debenham, the results of his research being published in his book referred to above. Recent research has shown that the Redgrave line is descended from the Sapiston line in the 17th century, and is the line of a family of Debenhams who became well-known photographers in London, the Isle of Wight and a number of other places in Britain. More about this family may be found here: Photography and the Debenhams.