Have to capitalise?

Client spent about £5k during the year on materials to build an office in his home garden, it is an asset of the company and he personal is charging a small notional amount to the company for electricity etc - no other uohao costs being claimed - in case anyone is wondering!

Simply, he is saying he never purchased an asset but materials which he has put together to form the office so does not want it on the balance sheet he just wants it w/o to the p+l. Obvioulsy for tax it will on the whole not be allowable but this is more of an accounting principle - can it be written off?

(The company is small, employs 2 staff building websites and turns over £200k - only other assets are furniture and computers)

Thoughts would be welcome