The garden in Newbury, Hampshire, was created by landscape and garden designer Daniel Lobb and funded by a £100,000 Heritage Lottery Fund grant. It was designed to complement the modernist proportions of the chapel itself, which is the only National Trust building dedicated to the Great War.
The garden will be a tranquil space for visitors, featuring fruit trees, cottage garden scented planting and a vegetable plot.
The opening will also mark the return to Sandham's chapel of Stanley Spencer's 19 large-scale paintings that also commemorate the forgotten dead.
Meanwhile, National Trust director-general Dame Helen Ghosh has visited Rosemary Verey's Barnsley House garden in Gloucestershire after she heard Sir Roy Strong speak at the trust's gardens conference last year and "being a garden enthusiast took up a recent offer to visit the gardens and learn about Sir Roy's interpretation of the property. It's always good to visit other gardens to compare and contrast with what the National Trust is doing."