What do you mean by refurbishment? Depending on what kind of work you are talking about getting done there might not be any tax deduction available. Structural alterations would be capital enhancements of the house and only allowed against CGT further down the line. Expenditure on fixtures & fittings may qualify for capital allowances.
In any case, in order to obtain tax relief the test is "wholly and exclusively"; so can you say categorically that the part of your house that is used for business is used wholly and exclusively for business? There may not be any issue with claiming a portion of utility bills on the basis that operating the business has increased these costs, but claiming refurbishment costs could open a couple of cans of worms...
Are you in receipt of mortgage interest relief at present on a mortgage on your home? If so then you would be immediately triggering a clawback of part of this relief, if as you say only 75% of your use of the house is as your PPR. As pointed out earlier, it would also affect your exemption from CGT on a future sale of the house.
If you are officially using part of your home as a commercial premises then there may be other issues such as commercial rates & insurance considerations, so I would tread very carefully and seek proper advice before plunging into anything here.