While architectural drawings form an important foundation of your budget for a house extension, loft conversion or renovation, they represent only a fraction of the total pre-construction costs.
Indeed, moving a project from concept to approval can introduce several necessary expenses – often including statutory fees and specialised technical assessments required to verify regulatory compliance, structural safety, and buildability.
Factoring in such expenses in your initial financial planning is a smart way to protect yourself from alarming budget surprises. Of course, no two projects are identical, so your final costs will depend entirely on your specific property, the complexity of the design, and any unique site constraints.
Planning application fees
Allow for the statutory cost of submitting your application
If your project requires planning permission, the local planning authority will usually charge an application fee. In England, the standard fee for alterations or extensions to a single dwellinghouse is, as of the 1st April 2026, £548.
Applications submitted online through the Planning Portal can also attract a service charge. For applications with a fee above £100, the current service charge is £75.83 plus VAT. Planning fees are subject to change, so the current figure should always be checked at the point of submission.
Some projects may fall within permitted development rights and follow a different application route, such as a Certificate of Lawful Development or prior approval. Listed buildings, conservation areas, flood zones and Green Belt locations can also affect the route and supporting information required.
Seeking expert advice at the earliest possible stage can help establish the likely approach before significant design work begins. Maidenhead Planning considers all the likely implications during the design stage and can manage the planning application process on a client’s behalf.
Building control fees
Planning approval and building regulations are separate stages
Planning permission deals with the acceptability of the proposed development. Building regulations, meanwhile, cover technical standards including structural safety, fire safety, thermal performance and drainage.
Most property extensions require building regulations approval. An application or inspection charge will generally apply, with local authority building control fees varying according to the nature and scale of the work. Standard charges are often available for common domestic projects, while larger or unusual schemes can be priced individually.
Homeowners are advised to allow for this separately from the planning application fee and the professional cost of producing technical drawings. A complete technical design package gives building control, engineers and contractors the information they need to assess and price the work.
Planning application fees
Allow for the statutory cost of submitting your application
If your project requires planning permission, the local planning authority will usually charge an application fee. In England, the standard fee for alterations or extensions to a single dwellinghouse is, as of the 1st April 2026, £548.
Applications submitted online through the Planning Portal can also attract a service charge. For applications with a fee above £100, the current service charge is £75.83 plus VAT. Planning fees are subject to change, so the current figure should always be checked at the point of submission.
Some projects may fall within permitted development rights and follow a different application route, such as a Certificate of Lawful Development or prior approval. Listed buildings, conservation areas, flood zones and Green Belt locations can also affect the route and supporting information required.
Seeking expert advice at the earliest possible stage can help establish the likely approach before significant design work begins. Maidenhead Planning considers all the likely implications during the design stage and can manage the planning application process on a client’s behalf.
Building control fees
Planning approval and building regulations are separate stages
Planning permission deals with the acceptability of the proposed development. Building regulations, meanwhile, cover technical standards including structural safety, fire safety, thermal performance and drainage.
Most property extensions require building regulations approval. An application or inspection charge will generally apply, with local authority building control fees varying according to the nature and scale of the work. Standard charges are often available for common domestic projects, while larger or unusual schemes can be priced individually.
Homeowners are advised to allow for this separately from the planning application fee and the professional cost of producing technical drawings. A complete technical design package gives building control, engineers and contractors the information they need to assess and price the work.
Structural engineering
Structural design can add a separate professional fee
Extensions, loft conversions and internal remodelling frequently involve structural work. Removing a load-bearing wall, forming a large opening, adding steelwork or altering the roof structure may require calculations and detailed input from a structural engineer.
The engineer’s fee will depend on the complexity of the structure and the amount of work required. Your architectural designer may coordinate this input and incorporate the engineer’s details into the technical drawing set, while the engineer provides their own specialist service.
Identifying structural requirements early on in the process can stop a project from progressing too far before a costly constraint is discovered. Accurate survey information, then, is extremely valuable, as existing wall thicknesses, levels and building geometry can all influence the structural solution.
Party wall input
Work near a shared boundary can create extra costs
The Party Wall etc. Act 1996 can apply to certain works involving a shared wall, work close to a neighbouring structure or excavation near an adjoining property.
Notices may need to be served before construction begins. If a dispute arises, surveyors can be appointed, and a Party Wall Award may be required. The owner carrying out the work will usually be responsible for the reasonable costs associated with the Award, although the exact allocation depends on the circumstances.
Party wall costs fall outside the planning application itself, so they can easily be overlooked during early budgeting. Raising the issue at the design stage gives you time to understand the process and account for possible surveyor fees.
Maidenhead Planning provides party wall support as part of its wider building surveying services and can advise on how the Act may affect a proposed project.
Specialist reports and site investigations
Your property can trigger additional supporting work
Planning and technical approvals sometimes require information from other specialists. A project could need an arboricultural report where protected trees are affected, a flood risk assessment in a relevant location, drainage information, contamination advice, ecology input or heritage guidance.
The requirement depends on the site and the proposal. Local planning policy and the validation requirements of the authority will influence the documents needed to support an application.
For projects with greater uncertainty, an initial planning appraisal can help identify relevant policies and areas of potential resistance before a full design is developed. Maidenhead Planning uses this process to assess feasibility before clients commit to further architectural costs.
Energy calculations and SAP assessments
Building performance can require specialist calculations
Energy compliance can introduce another professional cost. New dwellings require energy-performance calculations using an approved methodology at design stage and completion. Conversions creating new dwellings can also require SAP assessments, while some highly glazed extensions may need additional calculations to demonstrate compliance with Part L.
The cost will depend on the type of assessment and the information required. The calculations are normally prepared by a suitably qualified energy assessor and coordinated with the technical design.
Discussing glazing, insulation and energy performance early gives the design team more opportunity to develop a compliant solution before the drawings reach an advanced stage.
VAT, surveys and contingency
Build a budget around the whole project
Check each quotation carefully to see if VAT is included. Professional fees, approval charges and construction work can be presented in different ways. Most domestic extension and renovation work attracts VAT at the standard rate, so an ex-VAT figure can produce a noticeable difference once the full cost is calculated.
You should also allow for the survey and measured information needed to produce an accurate design. Maidenhead Planning uses detailed scanning and 3D drawing methods to capture the existing property, including intricacies such as wall thicknesses. The approach requires specialist equipment and software, while giving the team a more accurate starting point for the design and technical stages.
A separate contingency or project risk allowance is sensible, too. The appropriate amount depends on the age and condition of the property, the complexity of the work, and the level of uncertainty remaining. RICS guidance recommends linking risk allowances to the project's characteristics rather than relying on a generic figure for every build.
Start with a complete view of the budget
Early cost conversations make later decisions easier
A realistic project budget brings together design fees, planning charges, building control, engineering, specialist reports, surveys, VAT and an appropriate allowance for risk. Construction costs then sit within a clearer overall financial framework.
Maidenhead Planning offers a free 30-minute video consultation where you can discuss your project, the problem you are trying to solve and the likely approvals required. A staged fee proposal follows, covering the relevant design and approval services.
For clients seeking support in the later stages of their project, Maidenhead Planning offers a comprehensive design & build service that covers design, statutory approvals, contractor management, and construction.
Posted by Wouter De Jager on August 23rd 2026
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