With the Government’s 2025 Budget confirming that a new Mansion Tax on high-value residential properties will come into effect from April 2028, many self-builders are understandably asking how this will impact projects currently being designed or planned. For those creating homes expected to exceed the £2 million threshold, it’s important to understand how the tax is likely to be applied, when liability begins, and what considerations should be built into your early design decisions. self-build journey with clarity and confidence.
At Allan Corfield Architects, we work with clients who are often creating large one-off homes in desirable rural or commuter locations across the UK - areas where final property values can easily approach or exceed the £2 million threshold. Below we outline what mansion tax could mean for ongoing and upcoming self-build projects, and what you should be thinking about during the planning and design stages.
The short answer is: yes, potentially - depending on how the policy is applied in 2028.
The Budget announcement highlighted that the tax will be assessed based on the value of the completed property, not the point at which it received planning permission or began construction. This means:
Homes currently in planning or under construction will only become liable once complete and added to the valuation list
There is no automatic exemption for projects already underway unless future Treasury guidance introduces transitional relief
The tax will be recurring annually, similar to how higher council tax bands function
In effect, anyone planning a home expected to exceed £2 million should assume they may fall within the scope of the tax once the property is ready for occupation after April 2028
A property’s valuation is influenced by several factors beyond simple size or construction cost, including:
Location and postcode, particularly higher-value rural and commuter areas
Plot size and setting
Internal floor area and overall design
Specification level and finishes
Ancillary buildings, garages and leisure facilities
A well-designed home in a high-demand area may exceed the threshold with ease, while a larger home in a lower-value location might remain outside it.
Architects are not tax advisers, but we do have a responsibility to guide clients on project implications that could affect viability or long-term affordability. At ACA, our approach is proactive, transparent and grounded in practical design considerations.
We now advise all clients planning homes near or above the £2 million mark that:
A mansion tax will apply from April 2028
Liability will be based on the value at completion, not during design
We always encourage clients to obtain tailored advice from a qualified tax specialist.
An annual levy may be a manageable cost for some, but for others - particularly those financing larger builds or planning to sell shortly after completion - it could materially influence strategic decisions.
Clients should consider:
We never design homes specifically to avoid tax, but architects can help shape decisions that legitimately influence final valuation.
Possible considerations include:
Because valuation usually occurs when a property is deemed complete, some clients may choose to delay:
This can spread investment and control the point at which full valuation takes place.
In certain cases, designing a home as two linked dwellings may better fit lifestyle requirements while keeping each unit below the threshold.
For clarity and transparency, we record that:
The potential tax implications have been discussed
Clients are advised to obtain formal tax guidance
Design flexibility remains available
This ensures everyone proceeds with full awareness.
WHAT SHOULD SELF-BUILDERS DO NOW?
For anyone planning a high-value home expected to complete after April 2028, we recommend:
Proceed with design as normal - but stay aware of the confirmed policy landscape
Build flexibility into design where appropriate
Consult a tax specialist early in the process
Consider programme strategy, especially completion timing relative to April 2028
Focus on long-term value, such as energy performance, sustainability and build quality - features that retain worth regardless of taxation
The introduction of a mansion tax from April 2028 marks a significant shift for the UK’s high-value housing market. While it will affect some self-builders, it does not need to derail well-planned projects. With clear guidance, flexible design thinking and the right professional advice, clients can continue to create exceptional homes that suit their lifestyle, budget and long-term goals.
If you’re planning a self-build home that may exceed the £2 million threshold and would like to understand your design and valuation options, the ACA team is here to help.