Ashridge House, including raised terrace, walls and steps to east and south, and iron railings to north

Ashridge House is a substantial country house built between 1808 and 1821 to designs by James Wyatt and completed after his death by Sir Jeffry Wyatville. It is constructed of local Totternhoe ashlar stone with slate roofs.

The house faces north and is arranged in a picturesque composition along an east-west axis. The main block contains the state apartments. The east block houses private apartments and terminates in an orangery set at an angle. To the west, a former conservatory (now a dining room) and chapel overlook the gardens to the south, while offices and service rooms are arranged around two courts along the north front and to the west.

The house is designed in a picturesque Gothic style with asymmetrical elevations dominated by the chapel spire and a tower containing the staircase hall. It is characterised by numerous crenellated parapets and casement windows, mostly of two lights with arched glazing bars and Tudor-style dripmoulds.

The main block is symmetrical with a central tower. Jeffry Wyatville added enrichments and a porte-cochère to the north elevation. The porte-cochère has a pointed arch entrance of several moulded orders with intricately carved foliage in the spandrels and headstops representing Elizabeth I and Edward VI. It is flanked by octagonal turrets decorated with trefoil panels surmounted by crocketed finials. The tower behind has a projecting bay with crenellations and a wide pointed arch window with cinquefoil tracery set within trefoil panels. The family coat of arms is carved in relief above. On either side of the tower are four-bay blocks of three storeys plus a basement, lit by square-headed windows and terminating in broached octagonal towers. Decorative iron railings span the front of the main block.

The private family apartments to the east consist of a two-storey block of ten bays. The first three bays are set at a 45-degree angle to the main block. The following seven bays are again set at an angle and have at each end square tower bays with octagonal corner turrets rising above the parapets. These tower bays are lit on the ground floor by recessed pointed arch windows. The sixth and eighth bays are canted and embellished with a blind trefoil arcade in the parapets. Beyond is the long rear wall of the orangery, divided into seven bays by lesenes with trefoil panels rising into miniature octagonal turrets. Each bay is lit by a narrow window, some of which are blind. The orangery terminates in an octagonal tower bay with a battered base, lit on the ground floor by pointed arch windows and above by arrow slit windows. An array of gargoyles looks out from the string course beneath the parapet.

To the right of the main block is a two-storey range formerly containing offices such as the housekeeper's and steward's rooms. Stepped lesenes divide it into nine bays lit by square-headed casements with dripmoulds, with smaller windows above. A double-leaf door in the central bay has glazed upper panels with Gothic tracery. The three-storey square tower to the right, formerly the Evidence Room, is lit by a window on each floor and has a slightly projecting crenellated parapet supported by shaped brackets. This is followed by a long, recessed, single-storey service range divided by stepped lesenes into eleven bays lit by small lancet windows. In the projecting central bay, a depressed arch opening under a dripmould leads into the main service yard. Another three-storey square tower (formerly the laundry) projects forward, followed by a lower range known as Red Lodge, formerly used mainly for stable-related activities. Red Lodge is believed to contain surviving Tudor brickwork behind the Totternhoe stone facing, making it one of the oldest parts of the site. It is a symmetrical single-storey range with the same crenellated parapet as elsewhere, rising in the centre into a pediment. The central bay has a pointed arch doorway (with replaced door) and a two-light window above with a dripmould, flanked by windows in unadorned surrounds. There are two windows to the left and one to the right, followed by two pointed arch doorways, one into the building and the other into a side passage along the gateway into the stable yard. This dominant gateway takes the form of a triangular gable with a depressed arch opening flanked by broached octagonal turrets. A section of adjoining crenellated wall to the right terminates in an octagonal pier.

The east elevation consists of the main block, the private apartments, and the orangery. The main three-storey block has at each end a prominent octagonal broached tower and a projecting ground floor with a pierced crenellated parapet. The first bay is lit on the ground floor by a large 15-light window with wooden mullions and transoms (also presumably changed by Wyatville during his alterations), and an original window above. Another octagonal tower defines the library, which has a five-bay pointed arch loggia with quatrefoils in the spandrels and stepped lesenes. This was glazed in the second half of the 20th century to create more internal library space. The upper floors are lit by five windows of different sizes. The private apartments are arranged at an angle to the right in an irregular style. The first three single-storey bays are highly decorated with three-light windows, trefoil-shaped at the top and bottom, and blocked central panes bearing a shield. A Gothic niche consisting of an ogee arch canopy with crocketed finials and flanking octagonal columns with blind trefoil-headed panels contains a statue of Elizabeth I. This is a copy of the original statue by Sir Richard Westmacott, which was later removed. A prominent two-storey corner tower, lit on the ground floor by pointed arch windows with Gothic tracery, projects forward, after which the remaining private apartments are recessed at an angle. They have four irregular bays lit by mostly two-light windows, except for the first bay which has a much larger window flanked by octagonal turrets. The final element in this elevation is the single-storey orangery, divided into nine bays by stepped lesenes with wide canted bays at each end. The depressed arch openings have been blocked and new windows inserted. The crenellated parapet is embellished with blind trefoil arch panels.

The south elevation, which forms the third principal elevation overlooking the gardens, has the main block linked to the chapel on the left by the former conservatory (now a dining room), with service quarters beyond the chapel. The main block has two storeys and an attic just visible above the uncrenellated parapet, behind which rises the tower, lit by three pointed arch windows with cinquefoil tracery. The symmetrical façade has broached octagonal turrets at each end and a projecting central bay, all rising above the roofline. The central bay has a three-bay pointed arch, rib-vaulted loggia with columnettes and quatrefoils in the spandrels. On the inner wall are three niches delicately embellished with Gothic ornamentation. A canted oriel window above has cinquefoil-headed windows and blind arcading above and below. To each side are double-height canted bays with full-height windows with stone mullions and transoms, and blind arcading in the parapets. These light the former dining and drawing rooms. The canted bays are flanked by windows in the same style. The fenestration was changed in the 1860s. The adjoining former conservatory is divided into ten bays by stepped lesenes, the central three bays breaking forward. It has a pierced crenellated parapet behind which the mansard roof of the first-floor accommodation (added in 1929) is visible. The pointed arch openings in each bay have been blocked up with stone and pierced by six-light casements with wooden mullions and transoms.

The four-bay apsidal chapel in the Perpendicular style has a pierced crenellated parapet and square, stepped buttresses terminating in miniature square turrets. The pointed arch windows have two rows of cinquefoil tracery. The twice stepped-back, two-stage tower has blind arcading, pierced crenellated parapets and angle buttresses rising into crocketed octagonal finials. The first stage has alternating ogee arch windows and niches. The spire is a fibreglass replacement of the original. A raised terrace runs around the east and south sides of the house, accessed via numerous flights of steps lined by dwarf stone walls with square piers that have blind trefoil arch panels.

Adjoining the chapel at right angles to the left is the south side of the service court, a single-storey range with additional accommodation added above in the second half of the 20th century. It is divided by stepped lesenes into eleven bays, the middle four lit by windows under dripmoulds, with a doorway to the left leading into the service court. This is lined on three sides by red brick loggias forming covered walkways, and on the fourth side by a two-storey building in beige brick with three gables and a loggia, designed by Edward Buckton Lamb around 1859 to 1860. Following on is a three-storey square tower which housed the brewhouse, forming a pair with the tower on the north side. A short stretch of crenellated stone wall, forming the south side of a small courtyard, joins a stone gateway with a depressed arch and cross-gabled caps. At right angles to the gateway, forming the short west elevation of the house, is the west wall of the courtyard, constructed of red brick and flint and also crenellated. Adjoining the courtyard is the former open space of the drying ground, now infilled with the Brindley Lecture Hall built in 1983, which is not included in the listing. Following this is the two-storey rear elevation of Red Lodge, constructed of handmade red brick and lit by Tudor-style windows with stone mullions and transoms. A small single-storey extension in the same style was added to the south side of Red Lodge in 2009.

Inside, the rooms in the main block are arranged over three floors around a central staircase hall that rises the full height of the central tower. The state rooms are in both Gothic and classical styles and are as finely wrought and impressive as the external elevations.

The lofty entrance hall is faced in ashlar stone and rises through two storeys to a hammerbeam roof. At the far end a three-bay, rib-vaulted Gothic arch passageway supports a two-tier minstrels' gallery and leads into the dramatic staircase hall, which is the centrepiece of Wyatt's scheme. This is also in the Gothic style and faced in ashlar. The cantilevered Imperial stair has a cast iron balustrade and brass handrail. On the first floor the hall is surrounded by arcades of four-centred arches with niches in the corners containing statues by Sir Richard Westmacott of Ashridge's founders and benefactors. There are more tall niches above, then a corbelled-out gallery with a fine iron railing, and more arcading behind it carrying a fan-vaulted coving on which the lantern stands. This has a fan-vault in imitation of the crossing at Canterbury Cathedral and contained a dial connected to the external weather vane. The ceiling is constructed of an iron frame covered in plaster rather than stone.

The chapel is also in the Gothic style and characterised by Gothic detailing of exquisite delicacy. The antechapel opens up into the tower and leads through a tall arched opening into the main apsidal chapel, lit all around by tall windows. Fan-vaulted coving carries a canted panelled ceiling. The fixed pews along the east and west sides and the stone carved reredos incorporating canopies over the sedilia at the south end are of exceptional quality.

In the other state rooms, Wyatville favoured the classical style when he overhauled Wyatt's original schemes. The former billiard room (Hoskins Room), to the left of the entrance hall, is in the Renaissance style. The fireplace dates from 1350 and was imported by Lady Marian Alford especially for the room. A screen at the south end consisting of square columns is richly decorated in a plasterwork design of Renaissance motifs and has arched bookcase alcoves at each end, also richly ornate. This represents a very early use of fibrous plaster, patented by Leonard Desachy.

This room opens into the library which forms the east side of the main block. It is lined by arched niches containing bookcases of Macassar ebony inlaid with brass and has a coved ceiling with gilt rococo-style detailing. The saloon (Lady Marian Alford Room) is in a grand Italianate style. The door surrounds are colossal aedicules with pediments supported by gilt columns of rose serpentine marble from Italy. Two large marble fireplaces, based on those by Scamozzi in the Doge's Palace, were later given caryatids modelled on contemporary estate workers by the sculptor Mark Rogers Junior (1848 to 1933). The painted and gilded ceiling is also modelled on an Italian example in which the central panel is a copy of Guido Reni's Aurora of around 1600 from the Palazzo Respiglioso in Rome.

The former dining room (Wyatt Room) is in a style evocative of the mid-17th century with its carved Austrian oak wood panelling which incorporates the chimneypiece and the arms of the Brownlow family above the door. The deep moulded ceiling is enriched with classical motifs and was designed to represent the fruits of the earth, based on the scala d'oro in the Doge's Palace in Venice. Underneath the dining room and saloon is the twin-aisled undercroft to the great hall of the monastic refectory, dating from 1285 to 1350. The seven-bay undercroft has a rib-vaulted roof and much re-facing of the walls is likely to have taken place in the 19th century.

The private apartments in the east range contain former bedrooms, dressing rooms and sitting rooms. They are less grandly appointed but still impressive and retain much of their original fabric and fittings, including panelled doorcases and doors, panelled window shutters, ceiling cornices enriched with classical motifs and chimneypieces in various styles. More family bedrooms are arranged at mezzanine level around the staircase hall, leading off from vaulted corridors. Many retain six-panel doors, window shutters, decorative cornices and fireplaces. The former orangery at the far east of the house retains its iron support columns but the roof structure is hidden beneath a suspended ceiling.

The service wing to the west retains its general arrangement of ranges along the north and south fronts which are one room deep with internal courts, some of which have been infilled. The wing has been modernised and some smaller rooms have been knocked through to make larger rooms, but some historic joinery and fixtures survive, notably the fitted shelves and cupboards in the former Evidence Room and the kitchen range with its impressive stone Tudor-style surround. The brick vaulted well house beneath the chapel was dug out by the monks in the 13th century. The current superstructure is a donkey wheel which operated a three-stage pump, probably installed when the house was rebuilt in the early 19th century.