When valuing garden land, here are some things you should consider:

1. Location: The location of the garden land is an important factor in determining its value. Factors such as proximity to amenities, transport links, and schools can affect the value.
2. Size: The size of the garden land will also have an impact on its value. Larger plots of land generally command a higher price than smaller plots.
3. Planning Permission: If the land has planning permission for development, this will have a significant impact on its value. Garden land with planning permission is often more valuable than land without it.
4. Accessibility: The accessibility of the garden land is another factor that can affect its value. If the land is difficult to access, it may be less valuable than land that is easily accessible.
5. Local Market: The local property market will also influence the value of garden land. You should research the local area to determine the demand for garden land and the prices of similar plots.
6. Condition: The condition of the garden land is another important factor to consider. If the land is well-maintained and has good quality soil, it may be more valuable than land that is overgrown or in poor condition. 
7. Zoning: You should also check the zoning of the land to determine what it can be used for. For example, if the land is zoned for residential use, it may be more valuable than land zoned for agricultural use.
8. Utilities: The availability of utilities such as water, electricity, and gas can also affect the value of garden land. If the land has access to these services, it may be more valuable than land without them.
9. Legal Considerations: Finally, you should consider any legal considerations that may affect the value of the land, such as easements or rights of way. It's important to consult with a solicitor or legal professional to ensure you are aware of any legal issues that may impact the value of the land.

There are two main methods of valuing garden land in the UK - the comparable approach and the "marriage value" approach. The comparable approach involves analyzing data on sales of similar plots of land and making adjustments based on factors such as size, location, and scarcity.

The "marriage value" approach is based on the idea that the land adds value to the house it is being merged with, and the value of the land is determined by calculating how much it contributes to the overall value of the property. For example, if the land adds £10,000 to the value of the house, the value of the land would be reflected as a portion of that increase. Typically, the uplift is split equally between buyer and seller, resulting in a value of £5,000 for the land in the example.

Despite the straightforward theory behind valuing garden land, determining an exact value can be challenging. Access to reliable data on sales is often limited, transactions tend to be private, and there is typically no agent involved in the process. In the absence of an agent, buyers may pay more than the actual value of the land, making it necessary to carefully consider comparable evidence.

To avoid overpaying or underselling garden land, it is advisable to seek the guidance of a surveyor or land agent. These professionals can provide valuable insights and ensure that both buyers and sellers receive a fair deal.