Wild speculation aside... It is possible that an earlier building on the site, presumably of timber framed construction, might have been limewashed (a frequent occurrence) - but on that basis almost every mill in the country has probably been a white mill at some time in its life. A more likely explanation ties in with the fact that we have a former chalk pit (now the car park) behind the mill, and that the west end of the building appears to stand on an artificial island made largely from chalk. So it wouldn't just have been the mill that was white, the whole area would have been white from all the chalk. We shall probably never know for sure.
The present edition of the mill worked under water power until 1866 when a severe winter flood breached the diversionary works in the river so severely that they were deemed beyond economic repair. By this time the miller was also the local baker so, rather than simply closing the mill, he converted one half of it to run from a portable steam engine in order to keep his bakehouse supplied with flour. Commercial milling however appears to have ended with the flood.
With the retirement of the last miller, around the end of the Nineteenth century, the working life of the mill came to an end and the millstones came to rest. By this time the Joyce family were one of the oldest and most respected tenants of the Bankes' Kingston Lacy Estate.
In "A Kingston Lacy Childhood" Viola Bankes (the sister of Ralph Bankes who gave the Kingston Lacy estates to the National Trust in 1982) recalls "The oldest tenant farmer on the Bankes estate was Mr. Joyce who made the tenants' speech at the reception at Kingston Lacy when my farther brought my mother there as his bride. My grandfather later offered Mr. Joyce the freehold on his farm, 'White Mill', because of the long association of his family with the Bankes family, but the proud old man refused it, saying, 'Thank you, but a Joyce always pays for what he has'".
After the turn of the century, the tenancy changed hands a couple of times in quick succession and the building spent the next 85 years rotting away as little more than a farm shed. Whitemill, along with the rest of the Kingston Lacy estates, was bequeathed to the National Trust by Ralph Bankes in 1982, but it wasn't until 1994 that the Trust found the resources to begin the painstaking conservation of the property. The key word here is "conservation" rather than restoration - the mill today does not, and will never again, work at the task for which it was built. The mill cottage is now home to the custodian, one of only an handful of such "custodial tenancies" of National Trust properties.
Prior to 1175 the history of the mill becomes a bit more uncertain. The Domesday Book is a bit of a let down since it doesn't list the local mills by name, merely recording a total of 8 mills in the manor:
WIMBORNE (Minster), SHAPWICK, (Moor) CRICHEL, WIMBORNE (St.Giles). King Edward held them in lordship. It is not known how many hides are there because they did not pay tax before 1066. Land for 45 ploughs. In lordship 5 ploughs; 15 slaves. 63 villagers, 68 smallholders and 7 cottagers have 22 ploughs. 8 mills which pay 110s; meadow, 150 acres; pasture 6 leagues long and 3 leagues wide; woodland 5 leagues long and 1 league wide. 3 cobs; 30 pigs; 250 sheep; 44 goats.
This extended manor covers a large part of East Dorset (perhaps as much as 45,000 acres), and presumably formed the core of the future Kingston Lacy Estate. However, within the region described, there are not enough other known mills to make it certain that Whitemill, or its predecessor, is not one of them.
Before the Saxons, there was considerable Roman activity around Badbury Rings, a mile or so to the North of the mill, with the junction of several major roads. The Romans are thought to have built a town just outside the Rings which they called Vindocladia - The Place Of White Walls. Might this be an even older origin for the name Whitemill?
All these settlements would have needed the services of a mill, so why shouldn't it have been on the site of the present Whitemill?