The construction work at Lyveden, near Brigstock, will provide a new visitor driveway, car park, visitor centre, cafe and accessible pathways up to the iconic 'New Bield' lodge.
Lyveden Reconnected aims to bring the manor building, acquired in 2013 - at the bottom of the site - and surrounding grounds back together as original owner Sir Thomas Tresham had intended.
Dr Shona Johnston, operations manager at Lyveden, said: "We are really excited to have this opportunity to tell the story of Lyveden and to actually be able to use this building.
"It will allow people to walk through the landscape. It's great to see Lyveden have the resources and attention it deserves and this project will help preserve Lyveden for the future."
The 110-acre site is part of a medieval deer park once owned by Sir Thomas Tresham who lived at Rushton Hall and built Triangular Lodge and the Market House in Rothwell.
It is thought that Sir Thomas and his wife Meriel kept the lodge and the manor house as a summer retreat and for hunting parties.
Sir Thomas Tresham was a Catholic landowner who remained loyal to the Pope at a time when Protestantism was the official religion in Britain.
During his time under house arrest for his Catholic beliefs, Sir Thomas designed his gardens and lodge - full of religious symbolism.
Previously, visitors shared farm track to access the car park and existing facilities housed in a cottage near to the imposing unfinished architectural tribute to faith in the shape of a cross.
By spring 2020, visitors will be able to park at the bottom of the hill and then make their way through to a visitor welcome centre constructed of English oak.
From there guests will walk up to the manor house, previously not open to the public, with new cafe, indoor and outdoor dining areas, interpretation rooms and toilets.
A new pedestrian route will take visitors through the gardens and orchard up to the 'New Bield' or lodge.
Dr Johnston said: "We know there was an older building on the site but the present manor house is all that remains of what was once a much larger Jacobean manor house with many
"Unfortunately there is very little left that is of historical importance. It has been modernised - the original oak staircase was sold to the Ford Edsel House in America.
"We want to create a modern traditional interior using textures, patterns and colours in the design of our interpretation space.
"Visitors will be able to see a key, that was found by one of our volunteers on the site, that dates from the 16th century and we will display the Tresham family tree.
"The great chamber has a wonderful hotch-potch of oak floor boards from original 17th century to the Victorian to modern day replacements.
"Visitors will be able to see history under their feet of this really flexible multi-purpose space."
Presently, 110 volunteers work with four members of staff on the site but the trust will be looking to fill four to five new paid posts in the house, garden and welcome centre.
Dr Johnston added: "We are on schedule to complete the works for the opening in mid May.
"We want to encourage people from Kettering, Corby, Wellingborough and all the surrounding areas to come and see this national treasure right on your doorstep."
For updates on Reconnecting Lyveden or for volunteering opportunities go to the National Trust website.