Your objection will be much more effective if you quote the legislation and planning rules which are breached by the proposal. Here is a guide:

The proposal is in breach of the following planning policies:

1. The National Planning Policy Framework

Paragraph 207: In determining applications, local planning authorities should require an applicant to describe the significance of any heritage assets affected, including any contribution made by their setting.

The application fails to demonstrate a clear understanding of the significance of the house and especially its magnificent stucco interiors as conceived by Giacomo Leoni and Giuseppe Artari, which are affected by the proposals.

Paragraph 208: Local planning authorities should identify and assess the particular significance of any heritage asset that may be affected by a proposal (including by development affecting the setting of a heritage asset) taking account of the available evidence and any necessary expertise. They should take this into account when considering the impact of a proposal on a heritage asset, to avoid or minimise any conflict between the heritage asset’s conservation and any aspect of the proposal.

The proposal will have a severe impact on a heritage asset of the greatest significance. The proposal also harms the setting of the asset with intrusive structures and increased noise. The proposal is in conflict with the conservation of the house.

Paragraph 210: In determining applications, local planning authorities should take account of:
a) the desirability of sustaining and enhancing the significance of heritage assets
and putting them to viable uses consistent with their conservation;
b) the positive contribution that conservation of heritage assets can make to
sustainable communities including their economic vitality; and
c) the desirability of new development making a positive contribution to local
character and distinctiveness.

Clandon Park with its austere exterior and exuberant stucco interiors was an incalculably great asset to the local area, instilling a sense of local pride, drawing visitors from far away and offering a beautiful and prestigious venue for weddings and local events. It is a given that if the house were restored to a high standard, all these benefits would return. In addition, the restoration project itself would generate significant interest as well as employment, training opportunities and a lifeline for endangered conservation skills. The proposed scheme, on the other hand, will always be far less attractive as a place to visit or as a venue. The reduction in the value of the asset to the community and economy will be substantial and permanent.

Paragraph 213: Any harm to, or loss of, the significance of a designated heritage asset (from its alteration or destruction, or from development within its setting), should require clear and convincing justification. Substantial harm to or loss of:
a) grade II listed buildings, or grade II registered parks or gardens, should be exceptional;
b) assets of the highest significance, notably scheduled monuments, protected wreck sites, registered battlefields, grade I and II* listed buildings, grade I and II* registered parks and gardens, and World Heritage Sites, should be wholly exceptional.

Clandon Park House is clearly an asset of the greatest significance. The circumstances are not so exceptional as to warrant a failure to reinstate the most significant parts of the house.

Paragraph 214: Where a proposed development will lead to substantial harm to (or total loss of significance of) a designated heritage asset, local planning authorities should refuse consent, unless it can be demonstrated that the substantial harm or total loss is
necessary to achieve substantial public benefits that outweigh that harm or loss, or
all of the following apply:
a) the nature of the heritage asset prevents all reasonable uses of the site; and
b) no viable use of the heritage asset itself can be found in the medium term through
appropriate marketing that will enable its conservation; and
c) conservation by grant-funding or some form of not for profit, charitable or public
ownership is demonstrably not possible; and
d) the harm or loss is outweighed by the benefit of bringing the site back into use.

Restored to its former glory, Clandon Park House would be very much viable, as it was before the fire, as visitor attraction, event space and wedding venue. It is already owned by a charity which is eminently capable of carrying out a restoration project as it did at Uppark in Sussex. The benefit of the proposed scheme is less than the benefit of authentic restoration, and therefore the harm involved in this scheme is in no way justified.

Paragraph 215: Where a development proposal will lead to less than substantial harm to the significance of a designated heritage asset, this harm should be weighed against the public benefits of the proposal including, where appropriate, securing its optimum viable use.

The optimum viable use of the house is as a venue and visitor attraction after a high-quality restoration using authentic materials and techniques. Without restoration and with unsympathetic and incongruous additions the house will be of less value as a visitor attraction and venue. It will be more difficult to attract paying visitors and will be less attractive and prestigious as a venue for events.

2. Guildford Borough Council’s Local Plan adopted in 2019

Local Plan Policy D18 1) (b) states that the planning application must include a Heritage Statement which demonstrates a clear understanding of the asset’s significance including all those parts affected by the proposals, and where applicable the contribution made to its setting.

Before the fire the house had a great significance as containing a series of outstanding interiors, the Marble Hall being one of the finest interiors in Europe of its date. With high-quality restoration of the architect’s conception the house can have this significance again. The proposal does not take account of the unique and high significance of the interiors and makes greatly exaggerated claims for 'a new significance' of the fire-damaged building. Had Clandon’s ruin commemorated some famous historical event such as bombing in the Second World War or the attack on the Twin Towers, then it could be said to have a ‘new significance’ but the fact is that it is a monument to nothing more than an electrical fault.

Local Plan Policy D18 1) the planning application must include a Heritage Statement which (c) explains how the asset and its setting will be affected by the proposal, including how the proposal preserves and enhances the heritage asset and better reveals its significance,
A prestigious and ambitious restoration project would realise the designs of the architect Leoni and the stuccadore Artari once more and restore the significance of the building. The option to restore the interiors has been rejected by the National Trust, who have claimed, without evidence, that restoration is impossible. This is contradicted by the extensive photographic record which was made of the house before the fire, the archaeological evidence of surviving fragments, testimony of expert stucco artists, as well as many outstanding examples of authentic restoration which have been undertaken elsewhere in Britain and many parts of Europe.

Local Plan Policy D18 1) The planning application must include a Heritage Statement which

d) demonstrates what steps have been taken to avoid, minimise or mitigate any
resultant harm, and

(e) presents a justification for the proposals that explains why any harm is considered necessary or desirable.

The harm resulting from this proposal (loss of the important interiors, demolition of historic fabric, introduction of unsympathetic interventions, disruption of historic layout) is not necessary for the purpose of securing the identified public benefits, including the building’s long-term conservation as a visitor attraction. The identified public benefits do not outweigh the harm caused by the proposed scheme. There are demonstrably less harmful ways to bring about the building’s renovation and re-use as a visitor attraction, notably through the faithful restoration of the important interiors and original roof structure.

Local Plan Policy D19 (2) Repairs, alterations or extensions that affect the special interest of a listed building are expected to:

(b) have regard to the historic internal layout as well as the architectural and historic integrity that form part of the special interest of the building

(c) reinforce the intrinsic character of the building through the use of appropriate materials, details and building techniques

The new walkways, staircases, lift and treatment of door openings fail to preserve or enhance the special interest of the listed building, compounding the harm done by the fire. They are intrusive elements of unsympathetic design, which erode historic character, make the plan form less legible and diminish the building’s architectural and artistic interest, which is of international importance.

3. Planning (Listed Buildings and Conservation Areas) Act 1990

Section 16(2) In considering whether to grant listed building consent for any works the local planning authority or the Secretary of State shall have special regard to the desirability of preserving the building or its setting or any features of special architectural or historic interest which it possesses.

In proposing not to repair the damage done by the fire, the National Trust intends to make permanent the harm done by the fire and permanently reduce the architectural and historic significance of the building.

Section 72(1) special attention shall be paid to the desirability of preserving or enhancing the character or appearance of that area.

The proposal clearly does not preserve the character of the house, but makes permanent the damage wrought by the fire and compounds it with unsympathetic interventions.

And a few more points:

• The spaces ‘created by the fire’ within the building are unsuitable for displaying artworks and artefacts which require a carefully controlled environment. New partitions, whether temporary or permanent, and intrusive display cases will inevitably be necessary for the display of exhibits and artwork within the building. This will have an undesirable effect on the character of the interior and overall significance.

Section 16(2) of the Planning Listed Buildings and Conservation Areas Act 1990.

Paragraphs 210 and 214 of the National Planning Policy Framework

Local Plan Policy D19 (2) (b) & (c)

• To the exterior the proposed roof terrace, metal railings, roof access pavilions and lift overrun will do irreparable harm to the building’s skyline and silhouette, causing significant harm to the building’s outstanding architectural interest. The effect on key proximate and long-distance views of the listed building from all sides would be seriously harmful to its significance. The new roof structure will require the removal of a considerable amount of historic fabric in the form of fire break walls dating from the 1730s roof structure. These walls define the plan of the Marble Hall and Saloon at roof level and are afforded the highest significance within Purcell / Alan Baxter’s Conservation Plan and Heritage Impact Assessment. The removal of these structures would do serious harm to the listed building’s significance.

  Sections 66(1), 16(2) and 72(1) of the Planning (Listed Buildings and Conservation Areas) Act 1990

  National Planning Policy Framework Paragraphs 210, 214 and 215

  Local Plan Policy D19 (2) (a) (b) and (c)

  Local Plan Policy D20 (2) (b) and (c)

  Local Plan Policy D22 1) (a) and (b)

• The new staircases are configured entirely differently to the four original staircases. The historic configuration of the oak staircase, red stair and bachelor stairs would be obliterated by the Proposed Scheme, to the significant detriment of historic plan form and spatial hierarchy, as well as the legibility of room function and historic circulation. Besides the harm to architectural and evidential values, all historic interest associated with the social aspects of how the staircases would have been used in the original house would be lost. The new staircases drastically simplify the layout and make no attempt to draw on the historic character of the house, contrary to local policy D19 (2) (b) & (c).

  Section 16(2) of the Planning Listed Buildings and Conservation Areas Act 1990

  Paragraphs 210 and 214 of the National Planning Policy Framework

  Local Plan Policy D19 (2) (b) & (c)

• The views from the Palladio Room towards the Grotto, east facing views from the Saloon, State Bedroom, Green Drawing Room and Hunting Room towards the Maori Meeting House and gardens, west facing views from the Marble Hall and Morning Room across the park and north facing views from the Library towards Capability Brown’s lake and Inwood’s temple, will all be adversely impacted by the permanently ruinous state of the interior. The east facing views from the Blue China Room, State Dining Room and Green Damask Room will be lost altogether due to the failure to reinstate the ceilings at ground floor level.

  Section 16(2) 66(1) of the Planning Listed Buildings and Conservation Areas Act 1990

  Paragraphs 210 and 214 of the National Planning Policy Framework

  Local Plan Policy D19 (2) (b) & (c)

• The proposed means of heating the building via air source heat pumps will do significant harm to the setting and significance of the listed building by way of unwelcome visual intrusion and noise disturbance on the main public approach to the building. The applicant’s Noise Assessment identifies a Major Impact / Significant Observed Adverse Effect as a result of the air source heat pumps and extract fans to the basement level of the house. This is not acceptable in the setting of a grade I listed country house.

  Sections 66(1), 16(2) and 72(1) of the Planning (Listed Buildings and Conservation Areas) Act 1990

  National Planning Policy Framework Paragraphs 210, 214 and 215

  Local policy D19 2) (a) (b) and (c)

  Local policy D20 2) (b) and (c)

  Local policy D22 1) (a) and (b)

• The external changes to Clandon Park House at roof level in conjunction with the air source heat pump enclosure would have a detrimental impact on the setting of other heritage assets, including the Grotto to the south, Maori Meeting House and Ionic Temple.

  Sections 66(1), and 72(1) of the Planning (Listed Buildings and Conservation Areas) Act 1990

  National Planning Policy Framework Paragraphs 210 and 215

  Local Plan Policy D19 (2) (a) (b) and (c)

  Local Plan Policy D20 (2) (b) and (c)

  Local Plan Policy D22 1) (a) and (b)

• By extension these adverse visual and audible effects will have a harmful impact on the significance of grade II Registered Park and Garden and West Clandon Conservation Area, both of which are directly affected.

  Section 72(1) of the Planning (Listed Buildings and Conservation Areas) Act 1990

  National Planning Policy Framework Paragraphs 210 and 215

  Local Plan Policy D20 (2) (b) and (c)

  Local Plan Policy D22 1) (a) and (b)

• There are significant concerns over the long-term viability of the property as a managed ruin. The prospect of repeat visits to observe smoke blackened walls and plaster is low and unlikely to result in large numbers over an extended period. There is only a small garden at Clandon and the novelty of a fire damaged interior will quickly wear off.

• The emphasis on flexible, public floor-space for collections displays, exhibitions, performance, social events such as parties and dining, workshops and meetings, as well as non-historic views from the rooftop of the building undermines the National Trust’s overriding responsibility to promote the permanent preservation for the benefit of the Nation buildings of beauty and historic interest.

• The wall between B21 and B22 which previously enclosed the Bachelor Stair has been categorised as highly significant. The complete removal of this wall to provide an opening to the lift and new stair prioritises the new interventions above historic fabric and heritage significance. It further compounds the substantial harm caused by the fire and is not justified on the basis of an improved approach from the north entrance or a better connection between basement and ground, with views up through the stair towards the west façade. Again, the National Trust imposes an idealised ‘post-fire’ significance to the detriment of the grade I listed building.

  Section 16(2) of the Planning Listed Buildings and Conservation Areas Act 1990

  Paragraphs 210, 214 and 215 of the National Planning Policy Framework

  Local Plan Policy D19 (2) (b) & (c)